DBI - Database independent interface for Perl

  use DBI;

  @driver_names = DBI->available_drivers;
  %drivers      = DBI->installed_drivers;
  @data_sources = DBI->data_sources($driver_name, \%attr);

  $dbh = DBI->connect($data_source, $username, $auth, \%attr);

  $rv  = $dbh->do($statement);
  $rv  = $dbh->do($statement, \%attr);
  $rv  = $dbh->do($statement, \%attr, @bind_values);

  $ary_ref  = $dbh->selectall_arrayref($statement);
  $hash_ref = $dbh->selectall_hashref($statement, $key_field);

  $ary_ref  = $dbh->selectcol_arrayref($statement);
  $ary_ref  = $dbh->selectcol_arrayref($statement, \%attr);

  @row_ary  = $dbh->selectrow_array($statement);
  $ary_ref  = $dbh->selectrow_arrayref($statement);
  $hash_ref = $dbh->selectrow_hashref($statement);

  $sth = $dbh->prepare($statement);
  $sth = $dbh->prepare_cached($statement);

  $rc = $sth->bind_param($p_num, $bind_value);
  $rc = $sth->bind_param($p_num, $bind_value, $bind_type);
  $rc = $sth->bind_param($p_num, $bind_value, \%attr);

  $rv = $sth->execute;
  $rv = $sth->execute(@bind_values);
  $rv = $sth->execute_array(\%attr, ...);

  $rc = $sth->bind_col($col_num, \$col_variable);
  $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);

  @row_ary  = $sth->fetchrow_array;
  $ary_ref  = $sth->fetchrow_arrayref;
  $hash_ref = $sth->fetchrow_hashref;

  $ary_ref  = $sth->fetchall_arrayref;
  $ary_ref  = $sth->fetchall_arrayref( $slice, $max_rows );

  $hash_ref = $sth->fetchall_hashref( $key_field );

  $rv  = $sth->rows;

  $rc  = $dbh->begin_work;
  $rc  = $dbh->commit;
  $rc  = $dbh->rollback;

  $quoted_string = $dbh->quote($string);

  $rc  = $h->err;
  $str = $h->errstr;
  $rv  = $h->state;

  $rc  = $dbh->disconnect;

The synopsis above only lists the major methods and parameters.

If you have questions about DBI, or DBD driver modules, you can get help from the dbi-users@perl.org mailing list. You can get help on subscribing and using the list by emailing dbi-users-help@perl.org.

(To help you make the best use of the dbi-users mailing list, and any other lists or forums you may use, I strongly recommend that you read How To Ask Questions The Smart Way by Eric Raymond: <http://www.catb.org/~esr/faqs/smart-questions.html>)

The DBI home page at <http://dbi.perl.org/> is always worth a visit and includes an FAQ and links to other resources.

Before asking any questions, reread this document, consult the archives and read the DBI FAQ. The archives are listed at the end of this document and on the DBI home page. An FAQ is installed as a DBI::FAQ module so you can read it by executing CWperldoc DBI::FAQ. However the DBI::FAQ module is currently (2004) outdated relative to the online FAQ on the DBI home page.

This document often uses terms like references, objects, methods. If you're not familar with those terms then it would be a good idea to read at least the following perl manuals first: perlreftut, perldsc, perllol, and perlboot.

Please note that Tim Bunce does not maintain the mailing lists or the web page (generous volunteers do that). So please don't send mail directly to him; he just doesn't have the time to answer questions personally. The dbi-users mailing list has lots of experienced people who should be able to help you if you need it. If you do email Tim he's very likely to just forward it to the mailing list.

This is the DBI specification that corresponds to the DBI version 1.50.

The DBI is evolving at a steady pace, so it's good to check that you have the latest copy.

The significant user-visible changes in each release are documented in the DBI::Changes module so you can read them by executing CWperldoc DBI::Changes.

Some DBI changes require changes in the drivers, but the drivers can take some time to catch up. Newer versions of the DBI have added features that may not yet be supported by the drivers you use. Talk to the authors of your drivers if you need a new feature that's not yet supported.

Features added after DBI 1.21 (February 2002) are marked in the text with the version number of the DBI release they first appeared in.

Extensions to the DBI API often use the CWDBIx::* namespace. See Naming Conventions and Name Space. DBI extension modules can be found at <http://search.cpan.org/search?mode=module&query=DBIx>. And all modules related to the DBI can be found at <http://search.cpan.org/search?query=DBI&mode=all>.

The DBI is a database access module for the Perl programming language. It defines a set of methods, variables, and conventions that provide a consistent database interface, independent of the actual database being used.

It is important to remember that the DBI is just an interface. The DBI is a layer of glue between an application and one or more database driver modules. It is the driver modules which do most of the real work. The DBI provides a standard interface and framework for the drivers to operate within.

             |<- Scope of DBI ->|
                  .-.   .--------------.   .-------------.
  .-------.       | |---| XYZ Driver   |---| XYZ Engine  |
  | Perl  |       | |   `--------------'   `-------------'
  | script|  |A|  |D|   .--------------.   .-------------.
  | using |--|P|--|B|---|Oracle Driver |---|Oracle Engine|
  | DBI   |  |I|  |I|   `--------------'   `-------------'
  | API   |       | |...
  |methods|       | |... Other drivers
  `-------'       | |...
                  `-'

The API, or Application Programming Interface, defines the call interface and variables for Perl scripts to use. The API is implemented by the Perl DBI extension.

The DBI dispatches the method calls to the appropriate driver for actual execution. The DBI is also responsible for the dynamic loading of drivers, error checking and handling, providing default implementations for methods, and many other non-database specific duties.

Each driver contains implementations of the DBI methods using the private interface functions of the corresponding database engine. Only authors of sophisticated/multi-database applications or generic library functions need be concerned with drivers.

The following conventions are used in this document:

  $dbh    Database handle object
  $sth    Statement handle object
  $drh    Driver handle object (rarely seen or used in applications)
  $h      Any of the handle types above ($dbh, $sth, or $drh)
  $rc     General Return Code  (boolean: true=ok, false=error)
  $rv     General Return Value (typically an integer)
  @ary    List of values returned from the database, typically a row of data
  $rows   Number of rows processed (if available, else -1)
  $fh     A filehandle
  undef   NULL values are represented by undefined values in Perl
  \%attr  Reference to a hash of attribute values passed to methods

Note that Perl will automatically destroy database and statement handle objects if all references to them are deleted.

To use DBI, first you need to load the DBI module:

  use DBI;
  use strict;

(The CWuse strict; isn't required but is strongly recommended.)

Then you need to connect to your data source and get a handle for that connection:

  $dbh = DBI->connect($dsn, $user, $password,
                      { RaiseError => 1, AutoCommit => 0 });

Since connecting can be expensive, you generally just connect at the start of your program and disconnect at the end.

Explicitly defining the required CWAutoCommit behaviour is strongly recommended and may become mandatory in a later version. This determines whether changes are automatically committed to the database when executed, or need to be explicitly committed later.

The DBI allows an application to prepare statements for later execution. A prepared statement is identified by a statement handle held in a Perl variable. We'll call the Perl variable CW$sth in our examples.

The typical method call sequence for a CWSELECT statement is:

  prepare,
    execute, fetch, fetch, ...
    execute, fetch, fetch, ...
    execute, fetch, fetch, ...

for example:

  $sth = $dbh->prepare("SELECT foo, bar FROM table WHERE baz=?");

  $sth->execute( $baz );

  while ( @row = $sth->fetchrow_array ) {
    print "@row\n";
  }

The typical method call sequence for a non-CWSELECT statement is:

  prepare,
    execute,
    execute,
    execute.

for example:

  $sth = $dbh->prepare("INSERT INTO table(foo,bar,baz) VALUES (?,?,?)");

  while(<CSV>) {
    chomp;
    my ($foo,$bar,$baz) = split /,/;
        $sth->execute( $foo, $bar, $baz );
  }

The CWdo() method can be used for non repeated non-CWSELECT statement (or with drivers that don't support placeholders):

  $rows_affected = $dbh->do("UPDATE your_table SET foo = foo + 1");

To commit your changes to the database (when AutoCommit is off):

  $dbh->commit;  # or call $dbh->rollback; to undo changes

Finally, when you have finished working with the data source, you should disconnect from it:

  $dbh->disconnect;

The DBI does not have a concept of a current session. Every session has a handle object (i.e., a CW$dbh) returned from the CWconnect method. That handle object is used to invoke database related methods.

Most data is returned to the Perl script as strings. (Null values are returned as CWundef.) This allows arbitrary precision numeric data to be handled without loss of accuracy. Beware that Perl may not preserve the same accuracy when the string is used as a number.

Dates and times are returned as character strings in the current default format of the corresponding database engine. Time zone effects are database/driver dependent.

Perl supports binary data in Perl strings, and the DBI will pass binary data to and from the driver without change. It is up to the driver implementors to decide how they wish to handle such binary data.

Most databases that understand multiple character sets have a default global charset. Text stored in the database is, or should be, stored in that charset; if not, then that's the fault of either the database or the application that inserted the data. When text is fetched it should be automatically converted to the charset of the client, presumably based on the locale. If a driver needs to set a flag to get that behaviour, then it should do so; it should not require the application to do that.

Multiple SQL statements may not be combined in a single statement handle (CW$sth), although some databases and drivers do support this (notably Sybase and SQL Server).

Non-sequential record reads are not supported in this version of the DBI. In other words, records can only be fetched in the order that the database returned them, and once fetched they are forgotten.

Positioned updates and deletes are not directly supported by the DBI. See the description of the CWCursorName attribute for an alternative.

Individual driver implementors are free to provide any private functions and/or handle attributes that they feel are useful. Private driver functions can be invoked using the DBI CWfunc() method. Private driver attributes are accessed just like standard attributes.

Many methods have an optional CW\%attr parameter which can be used to pass information to the driver implementing the method. Except where specifically documented, the CW\%attr parameter can only be used to pass driver specific hints. In general, you can ignore CW\%attr parameters or pass it as CWundef.

The DBI package and all packages below it (CWDBI::*) are reserved for use by the DBI. Extensions and related modules use the CWDBIx:: namespace (see <http://www.perl.com/CPAN/modules/by-module/DBIx/>). Package names beginning with CWDBD:: are reserved for use by DBI database drivers. All environment variables used by the DBI or by individual DBDs begin with "CWDBI_ or CWDBD_".

The letter case used for attribute names is significant and plays an important part in the portability of DBI scripts. The case of the attribute name is used to signify who defined the meaning of that name and its values.

  Case of name  Has a meaning defined by
  ------------  ------------------------
  UPPER_CASE    Standards, e.g.,  X/Open, ISO SQL92 etc (portable)
  MixedCase     DBI API (portable), underscores are not used.
  lower_case    Driver or database engine specific (non-portable)

It is of the utmost importance that Driver developers only use lowercase attribute names when defining private attributes. Private attribute names must be prefixed with the driver name or suitable abbreviation (e.g., "CWora_ for Oracle, CWing_" for Ingres, etc).

Most DBI drivers require applications to use a dialect of SQL (Structured Query Language) to interact with the database engine. The Standards Reference Information section provides links to useful information about SQL.

The DBI itself does not mandate or require any particular language to be used; it is language independent. In ODBC terms, the DBI is in pass-thru mode, although individual drivers might not be. The only requirement is that queries and other statements must be expressed as a single string of characters passed as the first argument to the prepare or do methods.

For an interesting diversion on the real history of RDBMS and SQL, from the people who made it happen, see:

  http://ftp.digital.com/pub/DEC/SRC/technical-notes/SRC-1997-018-html/sqlr95.html

Follow the Full Contents then Intergalactic dataspeak links for the SQL history.

Some drivers support placeholders and bind values. Placeholders, also called parameter markers, are used to indicate values in a database statement that will be supplied later, before the prepared statement is executed. For example, an application might use the following to insert a row of data into the SALES table:

  INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)

or the following, to select the description for a product:

  SELECT description FROM products WHERE product_code = ?

The CW? characters are the placeholders. The association of actual values with placeholders is known as binding, and the values are referred to as bind values.

Note that the CW? is not enclosed in quotation marks, even when the placeholder represents a string. Some drivers also allow placeholders like CW:name and CW:n (e.g., CW:1, CW:2, and so on) in addition to CW?, but their use is not portable.

With most drivers, placeholders can't be used for any element of a statement that would prevent the database server from validating the statement and creating a query execution plan for it. For example:

  "SELECT name, age FROM ?"         # wrong (will probably fail)
  "SELECT name, ?   FROM people"    # wrong (but may not 'fail')

Also, placeholders can only represent single scalar values. For example, the following statement won't work as expected for more than one value:

  "SELECT name, age FROM people WHERE name IN (?)"    # wrong
  "SELECT name, age FROM people WHERE name IN (?,?)"  # two names

When using placeholders with the SQL CWLIKE qualifier, you must remember that the placeholder substitutes for the whole string. So you should use "CW... LIKE ? ..." and include any wildcard characters in the value that you bind to the placeholder.

NULL Values

Undefined values, or CWundef, are used to indicate NULL values. You can insert and update columns with a NULL value as you would a non-NULL value. These examples insert and update the column CWage with a NULL value:

  $sth = $dbh->prepare(qq{
    INSERT INTO people (fullname, age) VALUES (?, ?)
  });
  $sth->execute("Joe Bloggs", undef);

  $sth = $dbh->prepare(qq{
    UPDATE people SET age = ? WHERE fullname = ?
  });
  $sth->execute(undef, "Joe Bloggs");

However, care must be taken when trying to use NULL values in a CWWHERE clause. Consider:

  SELECT fullname FROM people WHERE age = ?

Binding an CWundef (NULL) to the placeholder will not select rows which have a NULL CWage! At least for database engines that conform to the SQL standard. Refer to the SQL manual for your database engine or any SQL book for the reasons for this. To explicitly select NULLs you have to say "CWWHERE age IS NULL".

A common issue is to have a code fragment handle a value that could be either CWdefined or CWundef (non-NULL or NULL) at runtime. A simple technique is to prepare the appropriate statement as needed, and substitute the placeholder for non-NULL cases:

  $sql_clause = defined $age? "age = ?" : "age IS NULL";
  $sth = $dbh->prepare(qq{
    SELECT fullname FROM people WHERE $sql_clause
  });
  $sth->execute(defined $age ? $age : ());

The following technique illustrates qualifying a CWWHERE clause with several columns, whose associated values (CWdefined or CWundef) are in a hash CW%h:

  for my $col ("age", "phone", "email") {
    if (defined $h{$col}) {
      push @sql_qual, "$col = ?";
      push @sql_bind, $h{$col};
    }
    else {
      push @sql_qual, "$col IS NULL";
    }
  }
  $sql_clause = join(" AND ", @sql_qual);
  $sth = $dbh->prepare(qq{
      SELECT fullname FROM people WHERE $sql_clause
  });
  $sth->execute(@sql_bind);

The techniques above call prepare for the SQL statement with each call to execute. Because calls to prepare() can be expensive, performance can suffer when an application iterates many times over statements like the above.

A better solution is a single CWWHERE clause that supports both NULL and non-NULL comparisons. Its SQL statement would need to be prepared only once for all cases, thus improving performance. Several examples of CWWHERE clauses that support this are presented below. But each example lacks portability, robustness, or simplicity. Whether an example is supported on your database engine depends on what SQL extensions it provides, and where it supports the CW? placeholder in a statement.

  0)  age = ?
  1)  NVL(age, xx) = NVL(?, xx)
  2)  ISNULL(age, xx) = ISNULL(?, xx)
  3)  DECODE(age, ?, 1, 0) = 1
  4)  age = ? OR (age IS NULL AND ? IS NULL)
  5)  age = ? OR (age IS NULL AND SP_ISNULL(?) = 1)
  6)  age = ? OR (age IS NULL AND ? = 1)

Statements formed with the above CWWHERE clauses require execute statements as follows. The arguments are required, whether their values are CWdefined or CWundef.

  0,1,2,3)  $sth->execute($age);
  4,5)      $sth->execute($age, $age);
  6)        $sth->execute($age, defined($age) ? 0 : 1);

Example 0 should not work (as mentioned earlier), but may work on a few database engines anyway (e.g. Sybase). Example 0 is part of examples 4, 5, and 6, so if example 0 works, these other examples may work, even if the engine does not properly support the right hand side of the CWOR expression.

Examples 1 and 2 are not robust: they require that you provide a valid column value xx (e.g. '~') which is not present in any row. That means you must have some notion of what data won't be stored in the column, and expect clients to adhere to that.

Example 5 requires that you provide a stored procedure (SP_ISNULL in this example) that acts as a function: it checks whether a value is null, and returns 1 if it is, or 0 if not.

Example 6, the least simple, is probably the most portable, i.e., it should work with with most, if not all, database engines.

Here is a table that indicates which examples above are known to work on various database engines:

                   -----Examples------
                   0  1  2  3  4  5  6
                   -  -  -  -  -  -  -
  Oracle 9         N  Y  N  Y  Y  ?  Y
  Informix IDS 9   N  N  N  Y  N  Y  Y
  MS SQL           N  N  Y  N  Y  ?  Y
  Sybase           Y  N  N  N  N  N  Y
  AnyData,DBM,CSV  Y  N  N  N  Y  Y* Y

* Works only because Example 0 works.

DBI provides a sample perl script that will test the examples above on your database engine and tell you which ones work. It is located in the ex/ subdirectory of the DBI source distribution, or here: <http://svn.perl.org/modules/dbi/trunk/ex/perl_dbi_nulls_test.pl> Please use the script to help us fill-in and maintain this table.

Performance

Without using placeholders, the insert statement shown previously would have to contain the literal values to be inserted and would have to be re-prepared and re-executed for each row. With placeholders, the insert statement only needs to be prepared once. The bind values for each row can be given to the CWexecute method each time it's called. By avoiding the need to re-prepare the statement for each row, the application typically runs many times faster. Here's an example:

  my $sth = $dbh->prepare(q{
    INSERT INTO sales (product_code, qty, price) VALUES (?, ?, ?)
  }) or die $dbh->errstr;
  while (<>) {
      chomp;
      my ($product_code, $qty, $price) = split /,/;
      $sth->execute($product_code, $qty, $price) or die $dbh->errstr;
  }
  $dbh->commit or die $dbh->errstr;

See execute and bind_param for more details.

The CWq{...} style quoting used in this example avoids clashing with quotes that may be used in the SQL statement. Use the double-quote like CWqq{...} operator if you want to interpolate variables into the string. See Quote and Quote-like Operators in perlop for more details.

See also the bind_columns method, which is used to associate Perl variables with the output columns of a CWSELECT statement.

In this section, we cover the DBI class methods, utility functions, and the dynamic attributes associated with generic DBI handles.

Constants representing the values of the SQL standard types can be imported individually by name, or all together by importing the special CW:sql_types tag.

The names and values of all the defined SQL standard types can be produced like this:

  foreach (@{ $DBI::EXPORT_TAGS{sql_types} }) {
    printf "%s=%d\n", $_, &{"DBI::$_"};
  }

These constants are defined by SQL/CLI, ODBC or both. CWSQL_BIGINT is (currently) omitted, because SQL/CLI and ODBC provide conflicting codes.

See the type_info, type_info_all, and bind_param methods for possible uses.

Note that just because the DBI defines a named constant for a given data type doesn't mean that drivers will support that data type.

The following methods are provided by the DBI class:

  ($scheme, $driver, $attr_string, $attr_hash, $driver_dsn) = DBI->parse_dsn($dsn)
      or die "Can't parse DBI DSN '$dsn'";

  $dbh = DBI->connect($data_source, $username, $password)
            or die $DBI::errstr;
  $dbh = DBI->connect($data_source, $username, $password, \%attr)
            or die $DBI::errstr;

  dbi:DriverName:database_name
  dbi:DriverName:database_name@hostname:port
  dbi:DriverName:database=database_name;host=hostname;port=port

  $ENV{DBI_AUTOPROXY};dsn=$data_source

  $dbh = DBI->connect($data_source, $user, $pass, {
        PrintError => 0,
        AutoCommit => 0
  });

  dbi:DriverName(PrintWarn=>1,PrintError=>0,Taint=>1):...

  $dbh = DBI->connect($data_source, $user, $pass, $driver);

  $dbh = DBI->connect_cached($data_source, $username, $password)
            or die $DBI::errstr;
  $dbh = DBI->connect_cached($data_source, $username, $password, \%attr)
            or die $DBI::errstr;

  DBI->connect_cached(..., { private_foo_cachekey => "Bar", ... });

  DBI->connect_cached(..., { private_foo_cachekey => __FILE__.__LINE__, ... });

  my $CachedKids_hashref = $dbh->{Driver}->{CachedKids};
  %$CachedKids_hashref = () if $CachedKids_hashref;

  @ary = DBI->available_drivers;
  @ary = DBI->available_drivers($quiet);

  %drivers = DBI->installed_drivers();

  DBI->installed_versions;
  @ary  = DBI->installed_versions;
  %hash = DBI->installed_versions;

  perl -MDBI -e 'DBI->installed_versions'

  @ary = DBI->data_sources($driver);
  @ary = DBI->data_sources($driver, \%attr);

  DBI->trace($trace_setting)
  DBI->trace($trace_setting, $trace_filename)
  $trace_setting = DBI->trace;

In addition to the DBI methods listed in the previous section, the DBI package also provides several utility functions.

These can be imported into your code by listing them in the CWuse statement. For example:

  use DBI qw(neat data_diff);

Alternatively, all these utility functions (except hash) can be imported using the CW:utils import tag. For example:

  use DBI qw(:utils);

  $description = data_string_desc($string);

  UTF8 off, ASCII, 42 characters 42 bytes
  UTF8 off, non-ASCII, 42 characters 42 bytes
  UTF8 on, non-ASCII, 4 characters 6 bytes
  UTF8 on but INVALID encoding, non-ASCII, 4 characters 6 bytes
  UTF8 off, undef

  $diff = data_string_diff($a, $b);

 Params a & b     Result
 ------------     ------
 'aaa', 'aaa'     ''
 'aaa', 'abc'     'Strings differ at index 2: a[2]=a, b[2]=b'
 'aaa', undef     'String b is undef, string a has 3 characters'
 'aaa', 'aa'      'String b truncated after 2 characters'

  $diff = data_diff($a, $b);
  $diff = data_diff($a, $b, $logical);

  a: UTF8 off, ASCII, 3 characters 3 bytes
  b: UTF8 on, non-ASCII, 3 characters 5 bytes
  Strings differ at index 2: a[2]=c, b[2]=\x{263A}

  $str = neat($value);
  $str = neat($value, $maxlen);

  $str = neat_list(\@listref, $maxlen, $field_sep);

  @bool = looks_like_number(@array);

  $hash_value = DBI::hash($buffer, $type);

Dynamic attributes are always associated with the last handle used (that handle is represented by CW$h in the descriptions below).

Where an attribute is equivalent to a method call, then refer to the method call for all related documentation.

Warning: these attributes are provided as a convenience but they do have limitations. Specifically, they have a short lifespan: because they are associated with the last handle used, they should only be used immediately after calling the method that sets them. If in any doubt, use the corresponding method call. Equivalent to CW$h->err. Equivalent to CW$h->errstr. Equivalent to CW$h->state. Equivalent to CW$h->rows. Please refer to the documentation for the rows method. Returns the DBI object handle used for the most recent DBI method call. If the last DBI method call was a DESTROY then CW$DBI::lasth will return the handle of the parent of the destroyed handle, if there is one.

The following methods can be used by all types of DBI handles.

  $rv = $h->err;

  $str = $h->errstr;

  $str = $h->state;

  $rv = $h->set_err($err, $errstr);
  $rv = $h->set_err($err, $errstr, $state);
  $rv = $h->set_err($err, $errstr, $state, $method);
  $rv = $h->set_err($err, $errstr, $state, $method, $rv);

  $h->trace($trace_settings);
  $h->trace($trace_settings, $trace_filename);
  $trace_settings = $h->trace;

  $h->trace_msg($message_text);
  $h->trace_msg($message_text, $min_level);

  $h->func(@func_arguments, $func_name) or die ...;

  $is_implemented = $h->can($method_name);

  $trace_settings_integer = $h->parse_trace_flags($trace_settings);

  $bit_flag = $h->parse_trace_flag($trace_flag_name);

  $rc = $h1->swap_inner_handle( $h2 );
  $rc = $h1->swap_inner_handle( $h2, $allow_reparent );

These attributes are common to all types of DBI handles.

Some attributes are inherited by child handles. That is, the value of an inherited attribute in a newly created statement handle is the same as the value in the parent database handle. Changes to attributes in the new statement handle do not affect the parent database handle and changes to the database handle do not affect existing statement handles, only future ones.

Attempting to set or get the value of an unknown attribute generates a warning, except for private driver specific attributes (which all have names starting with a lowercase letter).

Example:

  $h->{AttributeName} = ...;    # set/write
  ... = $h->{AttributeName};    # get/read

    sub show_child_handles {
        my ($h, $level) = @_;
        $level ||= 0;
        printf "%sh %s %s\n", $h->{Type}, "\t" x $level, $h;
        show_child_handles($_, $level + 1) 
            for (grep { defined } @{$h->{ChildHandles}});
    }

    my %drivers = DBI->installed_drivers();
    show_child_handles($_) for (values %drivers);

  DBD::Oracle::db execute warning: ... warning text here ...

  DBD::Oracle::db prepare failed: ... error text here ...

  DBD::Oracle::db prepare failed: ... error text here ...

  eval {
    ...
    $sth->execute();
    ...
  };
  if ($@) {
    # $sth->err and $DBI::err will be true if error was from DBI
    warn $@; # print the error
    ... # do whatever you need to deal with the error
  }

  {
    local $h->{RaiseError};  # localize and turn off for this block
    ...
  }

  use Carp;
  $h->{HandleError} = sub { confess(shift) };

  use Exception; # or your own favourite exception module
  $h->{HandleError} = sub { Exception->new('DBI')->raise($_[0]) };

  sub your_subroutine {
    my $previous_handler = $h->{HandleError};
    $h->{HandleError} = sub {
      return 1 if $previous_handler and &$previous_handler(@_);
      ... your code here ...
    };
  }

  $h->{HandleError} = sub { $_[0]=Carp::longmess($_[0]); 0; };

  $h->{HandleError} = sub {
    return 0 unless $_[0] =~ /^\S+ fetchrow_arrayref failed:/;
    return 0 unless $_[1]->err == 1234; # the error to 'hide'
    $h->set_err(undef,undef);   # turn off the error
    $_[2] = [ ... ];    # supply alternative return value
    return 1;
  };

  $dbh->{LongReadLen} = $dbh->selectrow_array(qq{
      SELECT MAX(OCTET_LENGTH(long_column_name))
      FROM table WHERE ...
  });
  $sth = $dbh->prepare(qq{
      SELECT long_column_name, ... FROM table WHERE ...
  });

  my $foo = $dbh->{private_yourmodname_foo} ||= { ... }; # WRONG

  my $foo = $dbh->{private_yourmodname_foo};
  $foo ||= $dbh->{private_yourmodname_foo} = { ... };

This section covers the methods and attributes associated with database handles.

The following methods are specified for DBI database handles:

  $new_dbh = $dbh->clone();
  $new_dbh = $dbh->clone(\%attr);

  %attribues_used = ( %original_attributes, %attr );

  @ary = $dbh->data_sources();
  @ary = $dbh->data_sources(\%attr);

  $rows = $dbh->do($statement)           or die $dbh->errstr;
  $rows = $dbh->do($statement, \%attr)   or die $dbh->errstr;
  $rows = $dbh->do($statement, \%attr, @bind_values) or die ...

  sub do {
      my($dbh, $statement, $attr, @bind_values) = @_;
      my $sth = $dbh->prepare($statement, $attr) or return undef;
      $sth->execute(@bind_values) or return undef;
      my $rows = $sth->rows;
      ($rows == 0) ? "0E0" : $rows; # always return true if no error
  }

  my $rows_deleted = $dbh->do(q{
      DELETE FROM table
      WHERE status = ?
  }, undef, 'DONE') or die $dbh->errstr;

  $rv = $dbh->last_insert_id($catalog, $schema, $table, $field);
  $rv = $dbh->last_insert_id($catalog, $schema, $table, $field, \%attr);

  @row_ary = $dbh->selectrow_array($statement);
  @row_ary = $dbh->selectrow_array($statement, \%attr);
  @row_ary = $dbh->selectrow_array($statement, \%attr, @bind_values);

  $ary_ref = $dbh->selectrow_arrayref($statement);
  $ary_ref = $dbh->selectrow_arrayref($statement, \%attr);
  $ary_ref = $dbh->selectrow_arrayref($statement, \%attr, @bind_values);

  $hash_ref = $dbh->selectrow_hashref($statement);
  $hash_ref = $dbh->selectrow_hashref($statement, \%attr);
  $hash_ref = $dbh->selectrow_hashref($statement, \%attr, @bind_values);

  $ary_ref = $dbh->selectall_arrayref($statement);
  $ary_ref = $dbh->selectall_arrayref($statement, \%attr);
  $ary_ref = $dbh->selectall_arrayref($statement, \%attr, @bind_values);

  $hash_ref = $dbh->selectall_hashref($statement, $key_field);
  $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr);
  $hash_ref = $dbh->selectall_hashref($statement, $key_field, \%attr, @bind_values);

  $ary_ref = $dbh->selectcol_arrayref($statement);
  $ary_ref = $dbh->selectcol_arrayref($statement, \%attr);
  $ary_ref = $dbh->selectcol_arrayref($statement, \%attr, @bind_values);

  # get array of id and name pairs:
  my $ary_ref = $dbh->selectcol_arrayref("select id, name from table", { Columns=>[1,2] });
  my %hash = @$ary_ref; # build hash from key-value pairs so $hash{$id} => name

  $sth = $dbh->prepare($statement)          or die $dbh->errstr;
  $sth = $dbh->prepare($statement, \%attr)  or die $dbh->errstr;

  $sth = $dbh->prepare_cached($statement)
  $sth = $dbh->prepare_cached($statement, \%attr)
  $sth = $dbh->prepare_cached($statement, \%attr, $if_active)

1: finish() will be called on the statement handle, but the warning is suppressed.

2: Disables any checking.

3: The existing active statement handle will be removed from the cache and a new statement handle prepared and cached in its place. This is the safest option because it doesn't affect the state of the old handle, it just removes it from the cache. [Added in DBI 1.40]

Here are some examples of CWprepare_cached:

  sub insert_hash {
    my ($table, $field_values) = @_;
    my @fields = sort keys %$field_values; # sort required
    my @values = @{$field_values}{@fields};
    my $sql = sprintf "insert into %s (%s) values (%s)",
        $table, join(",", @fields), join(",", ("?")x@fields);
    my $sth = $dbh->prepare_cached($sql);
    return $sth->execute(@values);
  }

  sub search_hash {
    my ($table, $field_values) = @_;
    my @fields = sort keys %$field_values; # sort required
    my @values = @{$field_values}{@fields};
    my $qualifier = "";
    $qualifier = "where ".join(" and ", map { "$_=?" } @fields) if @fields;
    $sth = $dbh->prepare_cached("SELECT * FROM $table $qualifier");
    return $dbh->selectall_arrayref($sth, {}, @values);
  }

Caveat emptor: This caching can be useful in some applications, but it can also cause problems and should be used with care. Here is a contrived case where caching would cause a significant problem:

  my $sth = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
  $sth->execute(...);
  while (my $data = $sth->fetchrow_hashref) {

    # later, in some other code called within the loop...
    my $sth2 = $dbh->prepare_cached('SELECT * FROM foo WHERE bar=?');
    $sth2->execute(...);
    while (my $data2 = $sth2->fetchrow_arrayref) {
      do_stuff(...);
    }
  }

In this example, since both handles are preparing the exact same statement, CW$sth2 will not be its own statement handle, but a duplicate of CW$sth returned from the cache. The results will certainly not be what you expect. Typically the the inner fetch loop will work normally, fetching all the records and terminating when there are no more, but now CW$sth is the same as CW$sth2 the outer fetch loop will also terminate. You'll know if you run into this problem because prepare_cached() will generate a warning by default (when CW$if_active is false). The cache used by prepare_cached() is keyed by both the statement and any attributes so you can also avoid this issue by doing something like:

  $sth = $dbh->prepare_cached("...", { dbi_dummy => __FILE__.__LINE__ });

which will ensure that prepare_cached only returns statements cached by that line of code in that source file.

  $rc  = $dbh->commit     or die $dbh->errstr;

Commit (make permanent) the most recent series of database changes if the database supports transactions and AutoCommit is off. If CWAutoCommit is on, then calling CWcommit will issue a commit ineffective with AutoCommit warning. See also Transactions in the FURTHER INFORMATION section below.

  $rc  = $dbh->rollback   or die $dbh->errstr;

Rollback (undo) the most recent series of uncommitted database changes if the database supports transactions and AutoCommit is off. If CWAutoCommit is on, then calling CWrollback will issue a rollback ineffective with AutoCommit warning. See also Transactions in the FURTHER INFORMATION section below.

  $rc  = $dbh->begin_work   or die $dbh->errstr;

Enable transactions (by turning CWAutoCommit off) until the next call to CWcommit or CWrollback. After the next CWcommit or CWrollback, CWAutoCommit will automatically be turned on again. If CWAutoCommit is already off when CWbegin_work is called then it does nothing except return an error. If the driver does not support transactions then when CWbegin_work attempts to set CWAutoCommit off the driver will trigger a fatal error. See also Transactions in the FURTHER INFORMATION section below.

  $rc = $dbh->disconnect  or warn $dbh->errstr;

Disconnects the database from the database handle. CWdisconnect is typically only used before exiting the program. The handle is of little use after disconnecting. The transaction behaviour of the CWdisconnect method is, sadly, undefined. Some database systems (such as Oracle and Ingres) will automatically commit any outstanding changes, but others (such as Informix) will rollback any outstanding changes. Applications not using CWAutoCommit should explicitly call CWcommit or CWrollback before calling CWdisconnect. The database is automatically disconnected by the CWDESTROY method if still connected when there are no longer any references to the handle. The CWDESTROY method for each driver should implicitly call CWrollback to undo any uncommitted changes. This is vital behaviour to ensure that incomplete transactions don't get committed simply because Perl calls CWDESTROY on every object before exiting. Also, do not rely on the order of object destruction during global destruction, as it is undefined. Generally, if you want your changes to be commited or rolled back when you disconnect, then you should explicitly call commit or rollback before disconnecting. If you disconnect from a database while you still have active statement handles (e.g., SELECT statement handles that may have more data to fetch), you will get a warning. The warning may indicate that a fetch loop terminated early, perhaps due to an uncaught error. To avoid the warning call the CWfinish method on the active handles.

  $rc = $dbh->ping;

Attempts to determine, in a reasonably efficient way, if the database server is still running and the connection to it is still working. Individual drivers should implement this function in the most suitable manner for their database engine. The current default implementation always returns true without actually doing anything. Actually, it returns "CW0 but true" which is true but zero. That way you can tell if the return value is genuine or just the default. Drivers should override this method with one that does the right thing for their type of database. Few applications would have direct use for this method. See the specialized Apache::DBI module for one example usage.

  $value = $dbh->get_info( $info_type );

Returns information about the implementation, i.e. driver and data source capabilities, restrictions etc. It returns CWundef for unknown or unimplemented information types. For example:

  $database_version  = $dbh->get_info(  18 ); # SQL_DBMS_VER
  $max_select_tables = $dbh->get_info( 106 ); # SQL_MAXIMUM_TABLES_IN_SELECT

See Standards Reference Information for more detailed information about the information types and their meanings and possible return values. The DBI::Const::GetInfoType module exports a CW%GetInfoType hash that can be used to map info type names to numbers. For example:

  $database_version = $dbh->get_info( $GetInfoType{SQL_DBMS_VER} );

The names are a merging of the ANSI and ODBC standards (which differ in some cases). See DBI::Const::GetInfoType for more details. Because some DBI methods make use of get_info(), drivers are strongly encouraged to support at least the following very minimal set of information types to ensure the DBI itself works properly:

 Type  Name                        Example A     Example B
 ----  --------------------------  ------------  ----------------
   17  SQL_DBMS_NAME               'ACCESS'      'Oracle'
   18  SQL_DBMS_VER                '03.50.0000'  '08.01.0721 ...'
   29  SQL_IDENTIFIER_QUOTE_CHAR   '`'           '"'
   41  SQL_CATALOG_NAME_SEPARATOR  '.'           '@'
  114  SQL_CATALOG_LOCATION        1             2

  $sth = $dbh->table_info( $catalog, $schema, $table, $type );
  $sth = $dbh->table_info( $catalog, $schema, $table, $type, \%attr );

Returns an active statement handle that can be used to fetch information about tables and views that exist in the database. The arguments CW$catalog, CW$schema and CW$table may accept search patterns according to the database/driver, for example: CW$table = '%FOO%'; Remember that the underscore character ('CW_') is a search pattern that means match any character, so 'FOO_%' is the same as 'FOO%' and 'FOO_BAR%' will match names like 'FOO1BAR'. The value of CW$type is a comma-separated list of one or more types of tables to be returned in the result set. Each value may optionally be quoted, e.g.:

  $type = "TABLE";
  $type = "'TABLE','VIEW'";

In addition the following special cases may also be supported by some drivers:

  $sth = $dbh->table_info('%', '', '');

If your driver doesn't support one or more of the selection filter parameters then you may get back more than you asked for and can do the filtering yourself. This method can be expensive, and can return a large amount of data. (For example, small Oracle installation returns over 2000 rows.) So it's a good idea to use the filters to limit the data as much as possible. The statement handle returned has at least the following fields in the order show below. Other fields, after these, may also be present. TABLE_CAT: Table catalog identifier. This field is NULL (CWundef) if not applicable to the data source, which is usually the case. This field is empty if not applicable to the table. TABLE_SCHEM: The name of the schema containing the TABLE_NAME value. This field is NULL (CWundef) if not applicable to data source, and empty if not applicable to the table. TABLE_NAME: Name of the table (or view, synonym, etc). TABLE_TYPE: One of the following: TABLE, VIEW, SYSTEM TABLE, GLOBAL TEMPORARY, LOCAL TEMPORARY, ALIAS, SYNONYM or a type identifier that is specific to the data source. REMARKS: A description of the table. May be NULL (CWundef). Note that CWtable_info might not return records for all tables. Applications can use any valid table regardless of whether it's returned by CWtable_info. See also tables, Catalog Methods and Standards Reference Information.

  $sth = $dbh->column_info( $catalog, $schema, $table, $column );

Returns an active statement handle that can be used to fetch information about columns in specified tables. The arguments CW$schema, CW$table and CW$column may accept search patterns according to the database/driver, for example: CW$table = '%FOO%'; Note: The support for the selection criteria is driver specific. If the driver doesn't support one or more of them then you may get back more than you asked for and can do the filtering yourself. The statement handle returned has at least the following fields in the order shown below. Other fields, after these, may also be present. TABLE_CAT: The catalog identifier. This field is NULL (CWundef) if not applicable to the data source, which is often the case. This field is empty if not applicable to the table. TABLE_SCHEM: The schema identifier. This field is NULL (CWundef) if not applicable to the data source, and empty if not applicable to the table. TABLE_NAME: The table identifier. Note: A driver may provide column metadata not only for base tables, but also for derived objects like SYNONYMS etc. COLUMN_NAME: The column identifier. DATA_TYPE: The concise data type code. TYPE_NAME: A data source dependent data type name. COLUMN_SIZE: The column size. This is the maximum length in characters for character data types, the number of digits or bits for numeric data types or the length in the representation of temporal types. See the relevant specifications for detailed information. BUFFER_LENGTH: The length in bytes of transferred data. DECIMAL_DIGITS: The total number of significant digits to the right of the decimal point. NUM_PREC_RADIX: The radix for numeric precision. The value is 10 or 2 for numeric data types and NULL (CWundef) if not applicable. NULLABLE: Indicates if a column can accept NULLs. The following values are defined:

  SQL_NO_NULLS          0
  SQL_NULLABLE          1
  SQL_NULLABLE_UNKNOWN  2

REMARKS: A description of the column. COLUMN_DEF: The default value of the column. SQL_DATA_TYPE: The SQL data type. SQL_DATETIME_SUB: The subtype code for datetime and interval data types. CHAR_OCTET_LENGTH: The maximum length in bytes of a character or binary data type column. ORDINAL_POSITION: The column sequence number (starting with 1). IS_NULLABLE: Indicates if the column can accept NULLs. Possible values are: 'NO', 'YES' and ''. SQL/CLI defines the following additional columns:

  CHAR_SET_CAT
  CHAR_SET_SCHEM
  CHAR_SET_NAME
  COLLATION_CAT
  COLLATION_SCHEM
  COLLATION_NAME
  UDT_CAT
  UDT_SCHEM
  UDT_NAME
  DOMAIN_CAT
  DOMAIN_SCHEM
  DOMAIN_NAME
  SCOPE_CAT
  SCOPE_SCHEM
  SCOPE_NAME
  MAX_CARDINALITY
  DTD_IDENTIFIER
  IS_SELF_REF

Drivers capable of supplying any of those values should do so in the corresponding column and supply undef values for the others. Drivers wishing to provide extra database/driver specific information should do so in extra columns beyond all those listed above, and use lowercase field names with the driver-specific prefix (i.e., 'ora_...'). Applications accessing such fields should do so by name and not by column number. The result set is ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME and ORDINAL_POSITION. Note: There is some overlap with statement attributes (in perl) and SQLDescribeCol (in ODBC). However, SQLColumns provides more metadata. See also Catalog Methods and Standards Reference Information.

  $sth = $dbh->primary_key_info( $catalog, $schema, $table );

Returns an active statement handle that can be used to fetch information about columns that make up the primary key for a table. The arguments don't accept search patterns (unlike table_info()). For example:

  $sth = $dbh->primary_key_info( undef, $user, 'foo' );
  $data = $sth->fetchall_arrayref;

The statement handle will return one row per column, ordered by TABLE_CAT, TABLE_SCHEM, TABLE_NAME, and KEY_SEQ. If there is no primary key then the statement handle will fetch no rows. Note: The support for the selection criteria, such as CW$catalog, is driver specific. If the driver doesn't support catalogs and/or schemas, it may ignore these criteria. The statement handle returned has at least the following fields in the order shown below. Other fields, after these, may also be present. TABLE_CAT: The catalog identifier. This field is NULL (CWundef) if not applicable to the data source, which is often the case. This field is empty if not applicable to the table. TABLE_SCHEM: The schema identifier. This field is NULL (CWundef) if not applicable to the data source, and empty if not applicable to the table. TABLE_NAME: The table identifier. COLUMN_NAME: The column identifier. KEY_SEQ: The column sequence number (starting with 1). Note: This field is named ORDINAL_POSITION in SQL/CLI. PK_NAME: The primary key constraint identifier. This field is NULL (CWundef) if not applicable to the data source. See also Catalog Methods and Standards Reference Information.

  @key_column_names = $dbh->primary_key( $catalog, $schema, $table );

Simple interface to the primary_key_info() method. Returns a list of the column names that comprise the primary key of the specified table. The list is in primary key column sequence order. If there is no primary key then an empty list is returned.

  $sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
                               , $fk_catalog, $fk_schema, $fk_table );

  $sth = $dbh->foreign_key_info( $pk_catalog, $pk_schema, $pk_table
                               , $fk_catalog, $fk_schema, $fk_table
                               , \%attr );

Returns an active statement handle that can be used to fetch information about foreign keys in and/or referencing the specified table(s). The arguments don't accept search patterns (unlike table_info()). CW$pk_catalog, CW$pk_schema, CW$pk_table identify the primary (unique) key table (PKT). CW$fk_catalog, CW$fk_schema, CW$fk_table identify the foreign key table (FKT). If both PKT and FKT are given, the function returns the foreign key, if any, in table FKT that refers to the primary (unique) key of table PKT. (Note: In SQL/CLI, the result is implementation-defined.) If only PKT is given, then the result set contains the primary key of that table and all foreign keys that refer to it. If only FKT is given, then the result set contains all foreign keys in that table and the primary keys to which they refer. (Note: In SQL/CLI, the result includes unique keys too.) For example:

  $sth = $dbh->foreign_key_info( undef, $user, 'master');
  $sth = $dbh->foreign_key_info( undef, undef,   undef , undef, $user, 'detail');
  $sth = $dbh->foreign_key_info( undef, $user, 'master', undef, $user, 'detail');

Note: The support for the selection criteria, such as CW$catalog, is driver specific. If the driver doesn't support catalogs and/or schemas, it may ignore these criteria. The statement handle returned has the following fields in the order shown below. Because ODBC never includes unique keys, they define different columns in the result set than SQL/CLI. SQL/CLI column names are shown in parentheses. PKTABLE_CAT ( UK_TABLE_CAT ): The primary (unique) key table catalog identifier. This field is NULL (CWundef) if not applicable to the data source, which is often the case. This field is empty if not applicable to the table. PKTABLE_SCHEM ( UK_TABLE_SCHEM ): The primary (unique) key table schema identifier. This field is NULL (CWundef) if not applicable to the data source, and empty if not applicable to the table. PKTABLE_NAME ( UK_TABLE_NAME ): The primary (unique) key table identifier. PKCOLUMN_NAME (UK_COLUMN_NAME ): The primary (unique) key column identifier. FKTABLE_CAT ( FK_TABLE_CAT ): The foreign key table catalog identifier. This field is NULL (CWundef) if not applicable to the data source, which is often the case. This field is empty if not applicable to the table. FKTABLE_SCHEM ( FK_TABLE_SCHEM ): The foreign key table schema identifier. This field is NULL (CWundef) if not applicable to the data source, and empty if not applicable to the table. FKTABLE_NAME ( FK_TABLE_NAME ): The foreign key table identifier. FKCOLUMN_NAME ( FK_COLUMN_NAME ): The foreign key column identifier. KEY_SEQ ( ORDINAL_POSITION ): The column sequence number (starting with 1). UPDATE_RULE ( UPDATE_RULE ): The referential action for the UPDATE rule. The following codes are defined:

  CASCADE              0
  RESTRICT             1
  SET NULL             2
  NO ACTION            3
  SET DEFAULT          4

DELETE_RULE ( DELETE_RULE ): The referential action for the DELETE rule. The codes are the same as for UPDATE_RULE. FK_NAME ( FK_NAME ): The foreign key name. PK_NAME ( UK_NAME ): The primary (unique) key name. DEFERRABILITY ( DEFERABILITY ): The deferrability of the foreign key constraint. The following codes are defined:

  INITIALLY DEFERRED   5
  INITIALLY IMMEDIATE  6
  NOT DEFERRABLE       7

( UNIQUE_OR_PRIMARY ): This column is necessary if a driver includes all candidate (i.e. primary and alternate) keys in the result set (as specified by SQL/CLI). The value of this column is UNIQUE if the foreign key references an alternate key and PRIMARY if the foreign key references a primary key, or it may be undefined if the driver doesn't have access to the information. See also Catalog Methods and Standards Reference Information.

  @names = $dbh->tables( $catalog, $schema, $table, $type );
  @names = $dbh->tables;        # deprecated

Simple interface to table_info(). Returns a list of matching table names, possibly including a catalog/schema prefix. See table_info for a description of the parameters. If CW$dbh->get_info(2) returns true (29 is SQL_IDENTIFIER_QUOTE_CHAR) then the table names are constructed and quoted by quote_identifier to ensure they are usable even if they contain whitespace or reserved words etc. This means that the table names returned will include quote characters.

  $type_info_all = $dbh->type_info_all;

Returns a reference to an array which holds information about each data type variant supported by the database and driver. The array and its contents should be treated as read-only. The first item is a reference to an 'index' hash of CWName => CWIndex pairs. The items following that are references to arrays, one per supported data type variant. The leading index hash defines the names and order of the fields within the arrays that follow it. For example:

  $type_info_all = [
    {   TYPE_NAME         => 0,
        DATA_TYPE         => 1,
        COLUMN_SIZE       => 2,     # was PRECISION originally
        LITERAL_PREFIX    => 3,
        LITERAL_SUFFIX    => 4,
        CREATE_PARAMS     => 5,
        NULLABLE          => 6,
        CASE_SENSITIVE    => 7,
        SEARCHABLE        => 8,
        UNSIGNED_ATTRIBUTE=> 9,
        FIXED_PREC_SCALE  => 10,    # was MONEY originally
        AUTO_UNIQUE_VALUE => 11,    # was AUTO_INCREMENT originally
        LOCAL_TYPE_NAME   => 12,
        MINIMUM_SCALE     => 13,
        MAXIMUM_SCALE     => 14,
        SQL_DATA_TYPE     => 15,
        SQL_DATETIME_SUB  => 16,
        NUM_PREC_RADIX    => 17,
        INTERVAL_PRECISION=> 18,
    },
    [ 'VARCHAR', SQL_VARCHAR,
        undef, "'","'", undef,0, 1,1,0,0,0,undef,1,255, undef
    ],
    [ 'INTEGER', SQL_INTEGER,
        undef,  "", "", undef,0, 0,1,0,0,0,undef,0,  0, 10
    ],
  ];

More than one row may have the same value in the CWDATA_TYPE field if there are different ways to spell the type name and/or there are variants of the type with different attributes (e.g., with and without CWAUTO_UNIQUE_VALUE set, with and without CWUNSIGNED_ATTRIBUTE, etc). The rows are ordered by CWDATA_TYPE first and then by how closely each type maps to the corresponding ODBC SQL data type, closest first. The meaning of the fields is described in the documentation for the type_info method. An 'index' hash is provided so you don't need to rely on index values defined above. However, using DBD::ODBC with some old ODBC drivers may return older names, shown as comments in the example above. Another issue with the index hash is that the lettercase of the keys is not defined. It is usually uppercase, as show here, but drivers may return names with any lettercase. Drivers are also free to return extra driver-specific columns of information - though it's recommended that they start at column index 50 to leave room for expansion of the DBI/ODBC specification. The type_info_all() method is not normally used directly. The type_info method provides a more usable and useful interface to the data.

  @type_info = $dbh->type_info($data_type);

Returns a list of hash references holding information about one or more variants of CW$data_type. The list is ordered by CWDATA_TYPE first and then by how closely each type maps to the corresponding ODBC SQL data type, closest first. If called in a scalar context then only the first (best) element is returned. If CW$data_type is undefined or CWSQL_ALL_TYPES, then the list will contain hashes for all data type variants supported by the database and driver. If CW$data_type is an array reference then CWtype_info returns the information for the first type in the array that has any matches. The keys of the hash follow the same letter case conventions as the rest of the DBI (see Naming Conventions and Name Space). The following uppercase items should always exist, though may be undef:

TYPE_NAME (string)

Data type name for use in CREATE TABLE statements etc.

DATA_TYPE (integer)

SQL data type number.

COLUMN_SIZE (integer)

For numeric types, this is either the total number of digits (if the NUM_PREC_RADIX value is 10) or the total number of bits allowed in the column (if NUM_PREC_RADIX is 2). For string types, this is the maximum size of the string in characters. For date and interval types, this is the maximum number of characters needed to display the value.

LITERAL_PREFIX (string)

Characters used to prefix a literal. A typical prefix is "CW' for characters, or possibly CW0x" for binary values passed as hexadecimal. NULL (CWundef) is returned for data types for which this is not applicable.

LITERAL_SUFFIX (string)

Characters used to suffix a literal. Typically "CW'" for characters. NULL (CWundef) is returned for data types where this is not applicable.

CREATE_PARAMS (string)

Parameter names for data type definition. For example, CWCREATE_PARAMS for a CWDECIMAL would be "CWprecision,scale" if the DECIMAL type should be declared as CWDECIMAL(precision,scaleCW) where precision and scale are integer values. For a CWVARCHAR it would be "CWmax length". NULL (CWundef) is returned for data types for which this is not applicable.

NULLABLE (integer)

Indicates whether the data type accepts a NULL value: CW0 or an empty string = no, CW1 = yes, CW2 = unknown.

CASE_SENSITIVE (boolean)

Indicates whether the data type is case sensitive in collations and comparisons.

SEARCHABLE (integer)

Indicates how the data type can be used in a WHERE clause, as follows:

  0 - Cannot be used in a WHERE clause
  1 - Only with a LIKE predicate
  2 - All comparison operators except LIKE
  3 - Can be used in a WHERE clause with any comparison operator

UNSIGNED_ATTRIBUTE (boolean)

Indicates whether the data type is unsigned. NULL (CWundef) is returned for data types for which this is not applicable.

FIXED_PREC_SCALE (boolean)

Indicates whether the data type always has the same precision and scale (such as a money type). NULL (CWundef) is returned for data types for which this is not applicable.

AUTO_UNIQUE_VALUE (boolean)

Indicates whether a column of this data type is automatically set to a unique value whenever a new row is inserted. NULL (CWundef) is returned for data types for which this is not applicable.

LOCAL_TYPE_NAME (string)

Localized version of the CWTYPE_NAME for use in dialog with users. NULL (CWundef) is returned if a localized name is not available (in which case CWTYPE_NAME should be used).

MINIMUM_SCALE (integer)

The minimum scale of the data type. If a data type has a fixed scale, then CWMAXIMUM_SCALE holds the same value. NULL (CWundef) is returned for data types for which this is not applicable.

MAXIMUM_SCALE (integer)

The maximum scale of the data type. If a data type has a fixed scale, then CWMINIMUM_SCALE holds the same value. NULL (CWundef) is returned for data types for which this is not applicable.

SQL_DATA_TYPE (integer)

This column is the same as the CWDATA_TYPE column, except for interval and datetime data types. For interval and datetime data types, the CWSQL_DATA_TYPE field will return CWSQL_INTERVAL or CWSQL_DATETIME, and the CWSQL_DATETIME_SUB field below will return the subcode for the specific interval or datetime data type. If this field is NULL, then the driver does not support or report on interval or datetime subtypes.

SQL_DATETIME_SUB (integer)

For interval or datetime data types, where the CWSQL_DATA_TYPE field above is CWSQL_INTERVAL or CWSQL_DATETIME, this field will hold the subcode for the specific interval or datetime data type. Otherwise it will be NULL (CWundef). Although not mentioned explicitly in the standards, it seems there is a simple relationship between these values:

  DATA_TYPE == (10 * SQL_DATA_TYPE) + SQL_DATETIME_SUB

NUM_PREC_RADIX (integer)

The radix value of the data type. For approximate numeric types, CWNUM_PREC_RADIX contains the value 2 and CWCOLUMN_SIZE holds the number of bits. For exact numeric types, CWNUM_PREC_RADIX contains the value 10 and CWCOLUMN_SIZE holds the number of decimal digits. NULL (CWundef) is returned either for data types for which this is not applicable or if the driver cannot report this information.

INTERVAL_PRECISION (integer)

The interval leading precision for interval types. NULL is returned either for data types for which this is not applicable or if the driver cannot report this information. For example, to find the type name for the fields in a select statement you can do:

  @names = map { scalar $dbh->type_info($_)->{TYPE_NAME} } @{ $sth->{TYPE} }

Since DBI and ODBC drivers vary in how they map their types into the ISO standard types you may need to search for more than one type. Here's an example looking for a usable type to store a date:

  $my_date_type = $dbh->type_info( [ SQL_DATE, SQL_TIMESTAMP ] );

Similarly, to more reliably find a type to store small integers, you could use a list starting with CWSQL_SMALLINT, CWSQL_INTEGER, CWSQL_DECIMAL, etc. See also Standards Reference Information.

  $sql = $dbh->quote($value);
  $sql = $dbh->quote($value, $data_type);

Quote a string literal for use as a literal value in an SQL statement, by escaping any special characters (such as quotation marks) contained within the string and adding the required type of outer quotation marks.

  $sql = sprintf "SELECT foo FROM bar WHERE baz = %s",
                $dbh->quote("Don't");

For most database types, quote would return CW'Don''t' (including the outer quotation marks). An undefined CW$value value will be returned as the string CWNULL (without single quotation marks) to match how NULLs are represented in SQL. If CW$data_type is supplied, it is used to try to determine the required quoting behaviour by using the information returned by type_info. As a special case, the standard numeric types are optimized to return CW$value without calling CWtype_info. Quote will probably not be able to deal with all possible input (such as binary data or data containing newlines), and is not related in any way with escaping or quoting shell meta-characters. It is valid for the quote() method to return an SQL expression that evaluates to the desired string. For example:

  $quoted = $dbh->quote("one\ntwo\0three")

may return something like:

  CONCAT('one', CHAR(1), 'two', CHAR(0), 'three')

The quote() method should not be used with Placeholders and Bind Values.

  $sql = $dbh->quote_identifier( $name );
  $sql = $dbh->quote_identifier( $catalog, $schema, $table, \%attr );

Quote an identifier (table name etc.) for use in an SQL statement, by escaping any special characters (such as double quotation marks) it contains and adding the required type of outer quotation marks. Undefined names are ignored and the remainder are quoted and then joined together, typically with a dot (CW.) character. For example:

  $id = $dbh->quote_identifier( undef, 'Her schema', 'My table' );

would, for most database types, return CW"Her schema"."My table" (including all the double quotation marks). If three names are supplied then the first is assumed to be a catalog name and special rules may be applied based on what get_info returns for SQL_CATALOG_NAME_SEPARATOR (41) and SQL_CATALOG_LOCATION (114). For example, for Oracle:

  $id = $dbh->quote_identifier( 'link', 'schema', 'table' );

would return CW"schema"."table"@"link".

  $imp_data = $dbh->take_imp_data;

Leaves the CW$dbh in an almost dead, zombie-like, state and returns a binary string of raw implementation data from the driver which describes the current database connection. Effectively it detaches the underlying database API connection data from the DBI handle. After calling take_imp_data(), all other methods except CWDESTROY will generate a warning and return undef. Why would you want to do this? You don't, forget I even mentioned it. Unless, that is, you're implementing something advanced like a multi-threaded connection pool. See DBI::Pool. The returned CW$imp_data can be passed as a CWdbi_imp_data attribute to a later connect() call, even in a separate thread in the same process, where the driver can use it to 'adopt' the existing connection that the implementation data was taken from. Some things to keep in mind... * the CW$imp_data holds the only reference to the underlying database API connection data. That connection is still 'live' and won't be cleaned up properly unless the CW$imp_data is used to create a new CW$dbh which is then allowed to disconnect() normally. * using the same CW$imp_data to create more than one other new CW$dbh at a time may well lead to unpleasant problems. Don't do that. Any child statement handles are effectively destroyed when take_imp_data() is called. The CWtake_imp_data method was added in DBI 1.36 but wasn't useful till 1.49.

This section describes attributes specific to database handles.

Changes to these database handle attributes do not affect any other existing or future database handles.

Attempting to set or get the value of an unknown attribute generates a warning, except for private driver-specific attributes (which all have names starting with a lowercase letter).

Example:

  $h->{AutoCommit} = ...;       # set/write
  ... = $h->{AutoCommit};       # get/read

  Databases which don't support transactions at all.
  Databases in which a transaction is always active.
  Databases in which a transaction must be explicitly started (C<'BEGIN WORK'>).

  $dbh->{Driver}->{Name}

  0 - Automatically determine a reasonable cache size for each C<SELECT>
  1 - Disable the local row cache
 >1 - Cache this many rows
 <0 - Cache as many rows that will fit into this much memory for each C<SELECT>.

This section lists the methods and attributes associated with DBI statement handles.

The DBI defines the following methods for use on DBI statement handles:

  $sth->bind_param($p_num, $bind_value)
  $sth->bind_param($p_num, $bind_value, \%attr)
  $sth->bind_param($p_num, $bind_value, $bind_type)

  $dbh->{RaiseError} = 1;        # save having to check each method call
  $sth = $dbh->prepare("SELECT name, age FROM people WHERE name LIKE ?");
  $sth->bind_param(1, "John%");  # placeholders are numbered from 1
  $sth->execute;
  DBI::dump_results($sth);

  $sth->bind_param(1, $value, { TYPE => SQL_INTEGER });

  $sth->bind_param(1, $value, SQL_INTEGER);

  use DBI qw(:sql_types);

  $sth->bind_param(1, $value, SQL_DATETIME);

  INSERT INTO price(code, price) VALUES (?, CONVERT(MONEY,?))

  $rc = $sth->bind_param_inout($p_num, \$bind_value, $max_len)  or die $sth->errstr;
  $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, \%attr)     or ...
  $rv = $sth->bind_param_inout($p_num, \$bind_value, $max_len, $bind_type) or ...

  $rc = $sth->bind_param_array($p_num, $array_ref_or_value)
  $rc = $sth->bind_param_array($p_num, $array_ref_or_value, \%attr)
  $rc = $sth->bind_param_array($p_num, $array_ref_or_value, $bind_type)

  $dbh->{RaiseError} = 1;        # save having to check each method call
  $sth = $dbh->prepare("INSERT INTO staff (first_name, last_name, dept) VALUES(?, ?, ?)");
  $sth->bind_param_array(1, [ 'John', 'Mary', 'Tim' ]);
  $sth->bind_param_array(2, [ 'Booth', 'Todd', 'Robinson' ]);
  $sth->bind_param_array(3, "SALES"); # scalar will be reused for each row
  $sth->execute_array( { ArrayTupleStatus => \my @tuple_status } );

  $rv = $sth->execute                or die $sth->errstr;
  $rv = $sth->execute(@bind_values)  or die $sth->errstr;

  $rv = $sth->execute_array(\%attr) or die $sth->errstr;
  $rv = $sth->execute_array(\%attr, @bind_values) or die $sth->errstr;

  $sth = $dbh->prepare("INSERT INTO staff (first_name, last_name) VALUES (?, ?)");
  my $tuples = $sth->execute_array(
      { ArrayTupleStatus => \my @tuple_status },
      \@first_names,
      \@last_names,
  );
  if ($tuples) {
      print "Successfully inserted $tuples records\n";
  }
  else {
      for my $tuple (0..@last_names-1) {
          my $status = $tuple_status[$tuple];
          $status = [0, "Skipped"] unless defined $status;
          next unless ref $status;
          printf "Failed to insert (%s, %s): %s\n",
              $first_names[$tuple], $last_names[$tuple], $status->[1];
      }
  }

  $rc = $sth->execute_for_fetch($fetch_tuple_sub);
  $rc = $sth->execute_for_fetch($fetch_tuple_sub, \@tuple_status);

  my $sel = $dbh1->prepare("select foo, bar from table1");
  $sel->execute;

  my $ins = $dbh2->prepare("insert into table2 (foo, bar) values (?,?)");
  my $fetch_tuple_sub = sub { $sel->fetchrow_arrayref };

  my @tuple_status;
  $rc = $ins->execute_for_fetch($fetch_tuple_sub, \@tuple_status);
  my @errors = grep { ref $_ } @tuple_status;

  $ins->execute_for_fetch( sub { shift @array_of_arrays }, \@tuple_status);

  $ary_ref = $sth->fetchrow_arrayref;
  $ary_ref = $sth->fetch;    # alias

 @ary = $sth->fetchrow_array;

 $hash_ref = $sth->fetchrow_hashref;
 $hash_ref = $sth->fetchrow_hashref($name);

  $tbl_ary_ref = $sth->fetchall_arrayref;
  $tbl_ary_ref = $sth->fetchall_arrayref( $slice );
  $tbl_ary_ref = $sth->fetchall_arrayref( $slice, $max_rows  );

  $tbl_ary_ref = $sth->fetchall_arrayref([0]);

  $tbl_ary_ref = $sth->fetchall_arrayref([-2,-1]);

  $tbl_ary_ref = $sth->fetchall_arrayref({});

  $tbl_ary_ref = $sth->fetchall_arrayref({ foo=>1, BAR=>1 });

  my $rows = []; # cache for batches of rows
  while( my $row = ( shift(@$rows) || # get row from cache, or reload cache:
                     shift(@{$rows=$sth->fetchall_arrayref(undef,10_000)||[]}) )
  ) {
    ...
  }

  $hash_ref = $sth->fetchall_hashref($key_field);

  $dbh->{FetchHashKeyName} = 'NAME_lc';
  $sth = $dbh->prepare("SELECT FOO, BAR, ID, NAME, BAZ FROM TABLE");
  $sth->execute;
  $hash_ref = $sth->fetchall_hashref('id');
  print "Name for id 42 is $hash_ref->{42}->{name}\n";

  $sth = $dbh->prepare("SELECT foo, bar, baz FROM table");
  $sth->execute;
  $hash_ref = $sth->fetchall_hashref( [ qw(foo bar) ] );
  print "For foo 42 and bar 38, baz is $hash_ref->{42}->{38}->{baz}\n";

  $rc  = $sth->finish;

  SELECT foo FROM table WHERE bar=? ORDER BY foo

  $rv = $sth->rows;

  $rc = $sth->bind_col($column_number, \$var_to_bind);
  $rc = $sth->bind_col($column_number, \$var_to_bind, \%attr );
  $rc = $sth->bind_col($column_number, \$var_to_bind, $bind_type );

  $sth->bind_col(1, undef, { TYPE => SQL_DATETIME });

  $sth->bind_col(1, undef, SQL_DATETIME);

  use DBI qw(:sql_types);

  $rc = $sth->bind_columns(@list_of_refs_to_vars_to_bind);

  $dbh->{RaiseError} = 1; # do this, or check every call for errors
  $sth = $dbh->prepare(q{ SELECT region, sales FROM sales_by_region });
  $sth->execute;
  my ($region, $sales);

  # Bind Perl variables to columns:
  $rv = $sth->bind_columns(\$region, \$sales);

  # you can also use Perl's \(...) syntax (see perlref docs):
  #     $sth->bind_columns(\($region, $sales));

  # Column binding is the most efficient way to fetch data
  while ($sth->fetch) {
      print "$region: $sales\n";
  }

  $sth->execute;
  my %row;
  $sth->bind_columns( \( @row{ @{$sth->{NAME_lc} } } ));
  while ($sth->fetch) {
      print "$row{region}: $row{sales}\n";
  }

  $rows = $sth->dump_results($maxlen, $lsep, $fsep, $fh);

This section describes attributes specific to statement handles. Most of these attributes are read-only.

Changes to these statement handle attributes do not affect any other existing or future statement handles.

Attempting to set or get the value of an unknown attribute generates a warning, except for private driver specific attributes (which all have names starting with a lowercase letter).

Example:

  ... = $h->{NUM_OF_FIELDS};    # get/read

Some drivers cannot provide valid values for some or all of these attributes until after CW$sth->execute has been successfully called. Typically the attribute will be CWundef in these situations.

Some attributes, like NAME, are not appropriate to some types of statement, like SELECT. Typically the attribute will be CWundef in these situations.

See also finish to learn more about the effect it may have on some attributes. Number of fields (columns) in the data the prepared statement may return. Statements that don't return rows of data, like CWDELETE and CWCREATE set CWNUM_OF_FIELDS to 0. The number of parameters (placeholders) in the prepared statement. See SUBSTITUTION VARIABLES below for more details. Returns a reference to an array of field names for each column. The names may contain spaces but should not be truncated or have any trailing space. Note that the names have the letter case (upper, lower or mixed) as returned by the driver being used. Portable applications should use NAME_lc or NAME_uc.

  print "First column name: $sth->{NAME}->[0]\n";

  $sth = $dbh->prepare("select Id, Name from table");
  $sth->execute;
  @row = $sth->fetchrow_array;
  print "Name $row[ $sth->{NAME_lc_hash}{name} ]\n";

  ftp://sqlstandards.org/SC32/SQL_Registry/

  print "First column may return NULL\n" if $sth->{NULLABLE}->[0];

    DBD::Foo::db->install_method($method_name, \%attr);

Installs the driver-private method named by CW$method_name into the DBI method dispatcher so it can be called directly, avoiding the need to use the func() method. It is called as a static method on the driver class to which the method belongs. The method name must begin with the corresponding registered driver-private prefix. For example, for DBD::Oracle CW$method_name must being with 'CWora_', and for DBD::AnyData it must begin with 'CWad_'. The attributes can be used to provide fine control over how the DBI dispatcher handles the dispatching of the method. However, at this point, it's undocumented and very liable to change. (Volunteers to polish up and document the interface are very welcome to get in touch via dbi-dev@perl.org) Methods installed using install_method default to the standard error handling behaviour for DBI methods: clearing err and errstr before calling the method, and checking for errors to trigger RaiseError etc. on return. This differs from the default behaviour of func(). Note for driver authors: The DBD::Foo::xx->install_method call won't work until the class-hierarchy has been setup. Normally the DBI looks after that just after the driver is loaded. This means install_method() can't be called at the time the driver is loaded unless the class-hierarchy is set up first. The way to do that is to call the setup_driver() method:

    DBI->setup_driver('DBD::Foo');

An application can retrieve metadata information from the DBMS by issuing appropriate queries on the views of the Information Schema. Unfortunately, CWINFORMATION_SCHEMA views are seldom supported by the DBMS. Special methods (catalog methods) are available to return result sets for a small but important portion of that metadata:

  column_info
  foreign_key_info
  primary_key_info
  table_info

All catalog methods accept arguments in order to restrict the result sets. Passing CWundef to an optional argument does not constrain the search for that argument. However, an empty string ('') is treated as a regular search criteria and will only match an empty value.

Note: SQL/CLI and ODBC differ in the handling of empty strings. An empty string will not restrict the result set in SQL/CLI.

Most arguments in the catalog methods accept only ordinary values, e.g. the arguments of CWprimary_key_info(). Such arguments are treated as a literal string, i.e. the case is significant and quote characters are taken literally.

Some arguments in the catalog methods accept search patterns (strings containing '_' and/or '%'), e.g. the CW$table argument of CWcolumn_info(). Passing '%' is equivalent to leaving the argument CWundef.

Caveat: The underscore ('_') is valid and often used in SQL identifiers. Passing such a value to a search pattern argument may return more rows than expected! To include pattern characters as literals, they must be preceded by an escape character which can be achieved with

  $esc = $dbh->get_info( 14 );  # SQL_SEARCH_PATTERN_ESCAPE
  $search_pattern =~ s/([_%])/$esc$1/g;

The ODBC and SQL/CLI specifications define a way to change the default behaviour described above: All arguments (except list value arguments) are treated as identifier if the CWSQL_ATTR_METADATA_ID attribute is set to CWSQL_TRUE. Quoted identifiers are very similar to ordinary values, i.e. their body (the string within the quotes) is interpreted literally. Unquoted identifiers are compared in UPPERCASE.

The DBI (currently) does not support the CWSQL_ATTR_METADATA_ID attribute, i.e. it behaves like an ODBC driver where CWSQL_ATTR_METADATA_ID is set to CWSQL_FALSE.

Transactions are a fundamental part of any robust database system. They protect against errors and database corruption by ensuring that sets of related changes to the database take place in atomic (indivisible, all-or-nothing) units.

This section applies to databases that support transactions and where CWAutoCommit is off. See AutoCommit for details of using CWAutoCommit with various types of databases.

The recommended way to implement robust transactions in Perl applications is to use CWRaiseError and CWeval { ... } (which is very fast, unlike CWeval "..."). For example:

  $dbh->{AutoCommit} = 0;  # enable transactions, if possible
  $dbh->{RaiseError} = 1;
  eval {
      foo(...)        # do lots of work here
      bar(...)        # including inserts
      baz(...)        # and updates
      $dbh->commit;   # commit the changes if we get this far
  };
  if ($@) {
      warn "Transaction aborted because $@";
      # now rollback to undo the incomplete changes
      # but do it in an eval{} as it may also fail
      eval { $dbh->rollback };
      # add other application on-error-clean-up code here
  }

If the CWRaiseError attribute is not set, then DBI calls would need to be manually checked for errors, typically like this:

  $h->method(@args) or die $h->errstr;

With CWRaiseError set, the DBI will automatically CWdie if any DBI method call on that handle (or a child handle) fails, so you don't have to test the return value of each method call. See RaiseError for more details.

A major advantage of the CWeval approach is that the transaction will be properly rolled back if any code (not just DBI calls) in the inner application dies for any reason. The major advantage of using the CW$h->{RaiseError} attribute is that all DBI calls will be checked automatically. Both techniques are strongly recommended.

After calling CWcommit or CWrollback many drivers will not let you fetch from a previously active CWSELECT statement handle that's a child of the same database handle. A typical way round this is to connect the the database twice and use one connection for CWSELECT statements.

See AutoCommit and disconnect for other important information about transactions.

Many databases support blob (binary large objects), long, or similar datatypes for holding very long strings or large amounts of binary data in a single field. Some databases support variable length long values over 2,000,000,000 bytes in length.

Since values of that size can't usually be held in memory, and because databases can't usually know in advance the length of the longest long that will be returned from a CWSELECT statement (unlike other data types), some special handling is required.

In this situation, the value of the CW$h->{LongReadLen} attribute is used to determine how much buffer space to allocate when fetching such fields. The CW$h->{LongTruncOk} attribute is used to determine how to behave if a fetched value can't fit into the buffer.

See the description of LongReadLen for more information.

When trying to insert long or binary values, placeholders should be used since there are often limits on the maximum size of an CWINSERT statement and the quote method generally can't cope with binary data. See Placeholders and Bind Values.

Here's a complete example program to select and fetch some data:

  my $data_source = "dbi::DriverName:db_name";
  my $dbh = DBI->connect($data_source, $user, $password)
      or die "Can't connect to $data_source: $DBI::errstr";

  my $sth = $dbh->prepare( q{
          SELECT name, phone
          FROM mytelbook
  }) or die "Can't prepare statement: $DBI::errstr";

  my $rc = $sth->execute
      or die "Can't execute statement: $DBI::errstr";

  print "Query will return $sth->{NUM_OF_FIELDS} fields.\n\n";
  print "Field names: @{ $sth->{NAME} }\n";

  while (($name, $phone) = $sth->fetchrow_array) {
      print "$name: $phone\n";
  }
  # check for problems which may have terminated the fetch early
  die $sth->errstr if $sth->err;

  $dbh->disconnect;

Here's a complete example program to insert some data from a file. (This example uses CWRaiseError to avoid needing to check each call).

  my $dbh = DBI->connect("dbi:DriverName:db_name", $user, $password, {
      RaiseError => 1, AutoCommit => 0
  });

  my $sth = $dbh->prepare( q{
      INSERT INTO table (name, phone) VALUES (?, ?)
  });

  open FH, "<phone.csv" or die "Unable to open phone.csv: $!";
  while (<FH>) {
      chomp;
      my ($name, $phone) = split /,/;
      $sth->execute($name, $phone);
  }
  close FH;

  $dbh->commit;
  $dbh->disconnect;

Here's how to convert fetched NULLs (undefined values) into empty strings:

  while($row = $sth->fetchrow_arrayref) {
    # this is a fast and simple way to deal with nulls:
    foreach (@$row) { $_ = '' unless defined }
    print "@$row\n";
  }

The CWq{...} style quoting used in these examples avoids clashing with quotes that may be used in the SQL statement. Use the double-quote like CWqq{...} operator if you want to interpolate variables into the string. See Quote and Quote-like Operators in perlop for more details.

Perl 5.7 and later support a new threading model called iThreads. (The old 5.005 style threads are not supported by the DBI.)

In the iThreads model each thread has it's own copy of the perl interpreter. When a new thread is created the original perl interpreter is 'cloned' to create a new copy for the new thread.

If the DBI and drivers are loaded and handles created before the thread is created then it will get a cloned copy of the DBI, the drivers and the handles.

However, the internal pointer data within the handles will refer to the DBI and drivers in the original interpreter. Using those handles in the new interpreter thread is not safe, so the DBI detects this and croaks on any method call using handles that don't belong to the current thread (except for DESTROY).

Because of this (possibly temporary) restriction, newly created threads must make their own connctions to the database. Handles can't be shared across threads.

But BEWARE, some underlying database APIs (the code the DBD driver uses to talk to the database, often supplied by the database vendor) are not thread safe. If it's not thread safe, then allowing more than one thread to enter the code at the same time may cause subtle/serious problems. In some cases allowing more than one thread to enter the code, even if not at the same time, can cause problems. You have been warned.

Using DBI with perl threads is not yet recommended for production environments. For more information see <http://www.perlmonks.org/index.pl?node_id=288022>

Note: There is a bug in perl 5.8.2 when configured with threads and debugging enabled (bug #24463) which causes a DBI test to fail.

[The following only applies to systems with unix-like signal handling. I'd welcome additions for other systems, especially Windows.]

The first thing to say is that signal handling in Perl versions less than 5.8 is not safe. There is always a small risk of Perl crashing and/or core dumping when, or after, handling a signal because the signal could arrive and be handled while internal data structures are being changed. If the signal handling code used those same internal data structures it could cause all manner of subtle and not-so-subtle problems. The risk was reduced with 5.4.4 but was still present in all perls up through 5.8.0.

Beginning in perl 5.8.0 perl implements 'safe' signal handling if your system has the POSIX sigaction() routine. Now when a signal is delivered perl just makes a note of it but does not run the CW%SIG handler. The handling is 'defered' until a 'safe' moment.

Although this change made signal handling safe, it also lead to a problem with signals being defered for longer than you'd like. If a signal arrived while executing a system call, such as waiting for data on a network connection, the signal is noted and then the system call that was executing returns with an EINTR error code to indicate that it was interrupted. All fine so far.

The problem comes when the code that made the system call sees the EINTR code and decides it's going to call it again. Perl doesn't do that, but database code sometimes does. If that happens then the signal handler doesn't get called untill later. Maybe much later.

Fortunately there are ways around this which we'll discuss below. Unfortunately they make signals unsafe again.

The two most common uses of signals in relation to the DBI are for canceling operations when the user types Ctrl-C (interrupt), and for implementing a timeout using CWalarm() and CW$SIG{ALRM}.

Cancel

The DBI provides a CWcancel method for statement handles. The CWcancel method should abort the current operation and is designed to be called from a signal handler. For example:

  $SIG{INT} = sub { $sth->cancel };

However, few drivers implement this (the DBI provides a default method that just returns CWundef) and, even if implemented, there is still a possibility that the statement handle, and even the parent database handle, will not be usable afterwards. If CWcancel returns true, then it has successfully invoked the database engine's own cancel function. If it returns false, then CWcancel failed. If it returns CWundef, then the database driver does not have cancel implemented.

Timeout

The traditional way to implement a timeout is to set CW$SIG{ALRM} to refer to some code that will be executed when an ALRM signal arrives and then to call alarm($seconds) to schedule an ALRM signal to be delivered CW$seconds in the future. For example:

  eval {
    local $SIG{ALRM} = sub { die "TIMEOUT\n" };
    alarm($seconds);
    ... code to execute with timeout here ...
    alarm(0);  # cancel alarm (if code ran fast)
  };
  alarm(0);    # cancel alarm (if eval failed)
  if ( $@ eq "TIMEOUT" ) { ... }

Unfortunately, as described above, this won't always work as expected, depending on your perl version and the underlying database code. With Oracle for instance (DBD::Oracle), if the system which hosts the database is down the DBI->connect() call will hang for several minutes before returning an error.

The solution on these systems is to use the CWPOSIX::sigaction() routine to gain low level access to how the signal handler is installed.

The code would look something like this (for the DBD-Oracle connect()):

   use POSIX ':signal_h';

   my $mask = POSIX::SigSet->new( SIGALRM ); # signals to mask in the handler
   my $action = POSIX::SigAction->new( 
       sub { die "connect timeout" },        # the handler code ref
       $mask,
       # not using (perl 5.8.2 and later) 'safe' switch or sa_flags
   );
   my $oldaction = POSIX::SigAction->new();
   sigaction( 'ALRM', $action, $oldaction );
   my $dbh;
   eval {
      alarm(5); # seconds before time out
      $dbh = DBI->connect("dbi:Oracle:$dsn" ... );
      alarm(0); # cancel alarm (if connect worked fast)
   };
   alarm(0);    # cancel alarm (if eval failed)
   sigaction( 'ALRM', $oldaction );  # restore original signal handler
   if ( $@ ) ....

Similar techniques can be used for canceling statement execution.

Unfortunately, this solution is somewhat messy, and it does not work with perl versions less than perl 5.8 where CWPOSIX::sigaction() appears to be broken.

For a cleaner implementation that works across perl versions, see Lincoln Baxter's Sys::SigAction module at <http://search.cpan.org/~lbaxter/Sys-SigAction/>. The documentation for Sys::SigAction includes an longer discussion of this problem, and a DBD::Oracle test script.

Be sure to read all the signal handling sections of the perlipc manual.

And finally, two more points to keep firmly in mind. Firstly, remember that what we've done here is essentially revert to old style unsafe handling of these signals. So do as little as possible in the handler. Ideally just die(). Secondly, the handles in use at the time the signal is handled may not be safe to use afterwards.

DBI can be subclassed and extended just like any other object oriented module. Before we talk about how to do that, it's important to be clear about how the DBI classes and how they work together.

By default CW$dbh = DBI->connect(...) returns a CW$dbh blessed into the CWDBI::db class. And the CW$dbh->prepare method returns an CW$sth blessed into the CWDBI::st class (actually it simply changes the last four characters of the calling handle class to be CW::st).

The leading 'CWDBI' is known as the 'root class' and the extra 'CW::db' or 'CW::st' are the 'handle type suffixes'. If you want to subclass the DBI you'll need to put your overriding methods into the appropriate classes. For example, if you want to use a root class of CWMySubDBI and override the do(), prepare() and execute() methods, then your do() and prepare() methods should be in the CWMySubDBI::db class and the execute() method should be in the CWMySubDBI::st class.

To setup the inheritance hierarchy the CW@ISA variable in CWMySubDBI::db should include CWDBI::db and the CW@ISA variable in CWMySubDBI::st should include CWDBI::st. The CWMySubDBI root class itself isn't currently used for anything visible and so, apart from setting CW@ISA to include CWDBI, it should be left empty.

So, having put your overriding methods into the right classes, and setup the inheritance hierarchy, how do you get the DBI to use them? You have two choices, either a static method call using the name of your subclass:

  $dbh = MySubDBI->connect(...);

or specifying a CWRootClass attribute:

  $dbh = DBI->connect(..., { RootClass => 'MySubDBI' });

The only difference between the two is that using an explicit RootClass attribute will make the DBI automatically attempt to load a module by that name if the class doesn't exist.

If both forms are used then the attribute takes precedence.

When subclassing is being used then, after a successful new connect, the DBI->connect method automatically calls:

  $dbh->connected($dsn, $user, $pass, \%attr);

The default method does nothing. The call is made just to simplify any post-connection setup that your subclass may want to perform. The parameters are the same as passed to DBI->connect. If your subclass supplies a connected method, it should be part of the MySubDBI::db package.

One more thing to note: you must let the DBI do the handle creation. If you want to override the connect() method in your *::dr class then it must still call SUPER::connect to get a CW$dbh to work with. Similarly, an overridden prepare() method in *::db must still call SUPER::prepare to get a CW$sth. If you try to create your own handles using bless() then you'll find the DBI will reject them with an is not a DBI handle (has no magic) error.

Here's a brief example of a DBI subclass. A more thorough example can be found in t/subclass.t in the DBI distribution.

  package MySubDBI;

  use strict;

  use DBI;
  use vars qw(@ISA);
  @ISA = qw(DBI);

  package MySubDBI::db;
  use vars qw(@ISA);
  @ISA = qw(DBI::db);

  sub prepare {
    my ($dbh, @args) = @_;
    my $sth = $dbh->SUPER::prepare(@args)
        or return;
    $sth->{private_mysubdbi_info} = { foo => 'bar' };
    return $sth;
  }

  package MySubDBI::st;
  use vars qw(@ISA);
  @ISA = qw(DBI::st);

  sub fetch {
    my ($sth, @args) = @_;
    my $row = $sth->SUPER::fetch(@args)
        or return;
    do_something_magical_with_row_data($row)
        or return $sth->set_err(1234, "The magic failed", undef, "fetch");
    return $row;
  }

When calling a SUPER::method that returns a handle, be careful to check the return value before trying to do other things with it in your overridden method. This is especially important if you want to set a hash attribute on the handle, as Perl's autovivification will bite you by (in)conveniently creating an unblessed hashref, which your method will then return with usually baffling results later on. It's best to check right after the call and return undef immediately on error, just like DBI would and just like the example above.

If your method needs to record an error it should call the set_err() method with the error code and error string, as shown in the example above. The error code and error string will be recorded in the handle and available via CW$h->err and CW$DBI::errstr etc. The set_err() method always returns an undef or empty list as approriate. Since your method should nearly always return an undef or empty list as soon as an error is detected it's handy to simply return what set_err() returns, as shown in the example above.

If the handle has CWRaiseError, CWPrintError, or CWHandleError etc. set then the set_err() method will honour them. This means that if CWRaiseError is set then set_err() won't return in the normal way but will 'throw an exception' that can be caught with an CWeval block.

You can stash private data into DBI handles via CW$h->{private_..._*}. See the entry under ATTRIBUTES COMMON TO ALL HANDLES for info and important caveats.

The DBI has a powerful tracing mechanism built in. It enables you to see what's going on 'behind the scenes', both within the DBI and the drivers you're using.

Which details are written to the trace output is controlled by a combination of a trace level, an integer from 0 to 15, and a set of trace flags that are either on or off. Together these are known as the trace settings and are stored together in a single integer. For normal use you only need to set the trace level, and generally only to a value between 1 and 4.

Each handle has it's own trace settings, and so does the DBI. When you call a method the DBI merges the handles settings into its own for the duration of the call: the trace flags of the handle are OR'd into the trace flags of the DBI, and if the handle has a higher trace level then the DBI trace level is raised to match it. The previous DBI trace setings are restored when the called method returns.

Trace levels are as follows:

  0 - Trace disabled.
  1 - Trace DBI method calls returning with results or errors.
  2 - Trace method entry with parameters and returning with results.
  3 - As above, adding some high-level information from the driver
      and some internal information from the DBI.
  4 - As above, adding more detailed information from the driver.
  5 to 15 - As above but with more and more obscure information.

Trace level 1 is best for a simple overview of what's happening. Trace level 2 is a good choice for general purpose tracing. Levels 3 and above are best reserved for investigating a specific problem, when you need to see inside the driver and DBI.

The trace output is detailed and typically very useful. Much of the trace output is formatted using the neat function, so strings in the trace output may be edited and truncated by that function.

Trace flags are used to enable tracing of specific activities within the DBI and drivers. The DBI defines some trace flags and drivers can define others. DBI trace flag names begin with a capital letter and driver specific names begin with a lowercase letter, as usual.

Curently the DBI only defines two trace flags:

  ALL - turn on all DBI and driver flags (not recommended)
  SQL - trace SQL statements executed (not yet implemented)

The parse_trace_flags and parse_trace_flag methods are used to convert trace flag names into the coresponding integer bit flags.

The CW$h->trace method sets the trace settings for a handle and CWDBI->trace does the same for the DBI.

In addition to the trace method, you can enable the same trace information, and direct the output to a file, by setting the CWDBI_TRACE environment variable before starting Perl. See DBI_TRACE for more information.

Finally, you can set, or get, the trace settings for a handle using the CWTraceLevel attribute.

All of those methods use parse_trace_flags() and so allow you set both the trace level and multiple trace flags by using a string containing the trace level and/or flag names separated by vertical bar ("CW|) or comma (CW,") characters. For example:

  local $h->{TraceLevel} = "3|SQL|foo";

Initially trace output is written to CWSTDERR. Both the CW$h->trace and CWDBI->trace methods take an optional CW$trace_filename parameter. If specified, and can be opened in append mode, then all trace output (currently including that from other handles) is redirected to that file. A warning is generated if the file can't be opened.

Further calls to trace() without a CW$trace_filename do not alter where the trace output is sent. If CW$trace_filename is undefined, then trace output is sent to CWSTDERR and the previous trace file is closed.

Currently CW$trace_filename can't be a filehandle. But meanwhile you can use the special strings CW"STDERR" and CW"STDOUT" to select those filehandles.

You can add tracing to your own application code using the trace_msg method.

It can sometimes be handy to compare trace files from two different runs of the same script. However using a tool like CWdiff doesn't work well because the trace file is full of object addresses that may differ each run. Here's a handy little command to strip those out:

  perl -pe 's/\b0x[\da-f]{6,}/0xNNNN/gi; s/\b[\da-f]{6,}/<long number>/gi'

The DBI module recognizes a number of environment variables, but most of them should not be used most of the time. It is better to be explicit about what you are doing to avoid the need for environment variables, especially in a web serving system where web servers are stingy about which environment variables are available.

The DBI_DSN environment variable is used by DBI->connect if you do not specify a data source when you issue the connect. It should have a format such as dbi:Driver:databasename.

The DBI_DRIVER environment variable is used to fill in the database driver name in DBI->connect if the data source string starts dbi:: (thereby omitting the driver). If DBI_DSN omits the driver name, DBI_DRIVER can fill the gap.

The DBI_AUTOPROXY environment variable takes a string value that starts dbi:Proxy: and is typically followed by hostname=...;port=.... It is used to alter the behaviour of DBI->connect. For full details, see DBI::Proxy documentation.

The DBI_USER environment variable takes a string value that is used as the user name if the DBI->connect call is given undef (as distinct from an empty string) as the username argument. Be wary of the security implications of using this.

The DBI_PASS environment variable takes a string value that is used as the password if the DBI->connect call is given undef (as distinct from an empty string) as the password argument. Be extra wary of the security implications of using this.

The DBI_DBNAME environment variable takes a string value that is used only when the obsolescent style of DBI->connect (with driver name as fourth parameter) is used, and when no value is provided for the first (database name) argument.

The DBI_TRACE environment variable specifies the global default trace settings for the DBI at startup. Can also be used to direct trace output to a file. When the DBI is loaded it does:

  DBI->trace(split /=/, $ENV{DBI_TRACE}, 2) if $ENV{DBI_TRACE};

So if CWDBI_TRACE contains an "CW=" character then what follows it is used as the name of the file to append the trace to.

output appended to that file. If the name begins with a number followed by an equal sign (CW=), then the number and the equal sign are stripped off from the name, and the number is used to set the trace level. For example:

  DBI_TRACE=1=dbitrace.log perl your_test_script.pl

On Unix-like systems using a Bourne-like shell, you can do this easily on the command line:

  DBI_TRACE=2 perl your_test_script.pl

See TRACING for more information.

An old variable that should no longer be used; equivalent to DBI_TRACE.

The DBI_PROFILE environment variable can be used to enable profiling of DBI method calls. See DBI::Profile for more information.

The DBI_PUREPERL environment variable can be used to enable the use of DBI::PurePerl. See DBI::PurePerl for more information.

The CW$dbh handle you're using to call CWprepare is probably undefined because the preceding CWconnect failed. You should always check the return status of DBI methods, or use the RaiseError attribute. The CW$sth handle you're using to call CWexecute is probably undefined because the preceeding CWprepare failed. You should always check the return status of DBI methods, or use the RaiseError attribute.

DBI/DBD internal version mismatch

The DBD driver module was built with a different version of DBI than the one currently being used. You should rebuild the DBD module under the current version of DBI. (Some rare platforms require static linking. On those platforms, there may be an old DBI or DBD driver version actually embedded in the Perl executable being used.)

DBD driver has not implemented the AutoCommit attribute

The DBD driver implementation is incomplete. Consult the author. You attempted to set or get an unknown attribute of a handle. Make sure you have spelled the attribute name correctly; case is significant (e.g., Autocommit is not the same as AutoCommit).

A pure-perl emulation of the DBI is included in the distribution for people using pure-perl drivers who, for whatever reason, can't install the compiled DBI. See DBI::PurePerl.

Refer to the documentation for the DBD driver that you are using.

Refer to the SQL Language Reference Manual for the database engine that you are using.

More detailed information about the semantics of certain DBI methods that are based on ODBC and SQL/CLI standards is available on-line via microsoft.com, for ODBC, and www.jtc1sc32.org for the SQL/CLI standard:

 DBI method        ODBC function     SQL/CLI Working Draft
 ----------        -------------     ---------------------
 column_info       SQLColumns        Page 124
 foreign_key_info  SQLForeignKeys    Page 163
 get_info          SQLGetInfo        Page 214
 primary_key_info  SQLPrimaryKeys    Page 254
 table_info        SQLTables         Page 294
 type_info         SQLGetTypeInfo    Page 239

For example, for ODBC information on SQLColumns you'd visit:

  http://msdn.microsoft.com/library/en-us/odbc/htm/odbcsqlcolumns.asp

If that URL ceases to work then use the MSDN search facility at:

  http://search.microsoft.com/us/dev/

and search for CWSQLColumns returns using the exact phrase option. The link you want will probably just be called CWSQLColumns and will be part of the Data Access SDK.

And for SQL/CLI standard information on SQLColumns you'd read page 124 of the (very large) SQL/CLI Working Draft available from:

  http://www.jtc1sc32.org/sc32/jtc1sc32.nsf/Attachments/7E3B41486BD99C3488256B410064C877/$FILE/32N0744T.PDF

A hyperlinked, browsable version of the BNF syntax for SQL92 (plus Oracle 7 SQL and PL/SQL) is available here:

  http://cui.unige.ch/db-research/Enseignement/analyseinfo/SQL92/BNFindex.html

A BNF syntax for SQL3 is available here:

  http://www.sqlstandards.org/SC32/WG3/Progression_Documents/Informal_working_drafts/iso-9075-2-1999.bnf

The following links provide further useful information about SQL. Some of these are rather dated now but may still be useful.

  http://www.jcc.com/SQLPages/jccs_sql.htm
  http://www.contrib.andrew.cmu.edu/~shadow/sql.html
  http://www.altavista.com/query?q=sql+tutorial

Programming the Perl DBI, by Alligator Descartes and Tim Bunce. <http://books.perl.org/book/154>

Programming Perl 3rd Ed. by Larry Wall, Tom Christiansen & Jon Orwant. <http://books.perl.org/book/134>

Learning Perl by Randal Schwartz. <http://books.perl.org/book/101>

Details of many other books related to perl can be found at <http://books.perl.org>

Index of DBI related modules available from CPAN:

 http://search.cpan.org/search?mode=module&query=DBIx%3A%3A
 http://search.cpan.org/search?mode=doc&query=DBI

For a good comparison of RDBMS-OO mappers and some OO-RDBMS mappers (including Class::DBI, Alzabo, and DBIx::RecordSet in the former category and Tangram and SPOPS in the latter) see the Perl Object-Oriented Persistence project pages at:

 http://poop.sourceforge.net

A similar page for Java toolkits can be found at:

 http://c2.com/cgi-bin/wiki?ObjectRelationalToolComparison

The dbi-users mailing list is the primary means of communication among users of the DBI and its related modules. For details send email to:

 dbi-users-help@perl.org

There are typically between 700 and 900 messages per month. You have to subscribe in order to be able to post. However you can opt for a 'post-only' subscription.

Mailing list archives (of variable quality) are held at:

 http://groups.google.com/groups?group=perl.dbi.users
 http://www.xray.mpe.mpg.de/mailing-lists/dbi/
 http://www.mail-archive.com/dbi-users%40perl.org/

The DBI Home Page:

 http://dbi.perl.org/

Other DBI related links:

 http://tegan.deltanet.com/~phlip/DBUIdoc.html
 http://dc.pm.org/perl_db.html
 http://wdvl.com/Authoring/DB/Intro/toc.html
 http://www.hotwired.com/webmonkey/backend/tutorials/tutorial1.html
 http://bumppo.net/lists/macperl/1999/06/msg00197.html
 http://gmax.oltrelinux.com/dbirecipes.html

Other database related links:

 http://www.jcc.com/sql_stnd.html
 http://cuiwww.unige.ch/OSG/info/FreeDB/FreeDB.home.html

Security, especially the SQL Injection attack:

 http://www.ngssoftware.com/research/papers.html
 http://www.ngssoftware.com/papers/advanced_sql_injection.pdf
 http://www.ngssoftware.com/papers/more_advanced_sql_injection.pdf
 http://www.esecurityplanet.com/trends/article.php/2243461
 http://www.spidynamics.com/papers/SQLInjectionWhitePaper.pdf
 http://www.imperva.com/application_defense_center/white_papers/blind_sql_server_injection.html
 http://online.securityfocus.com/infocus/1644

Commercial and Data Warehouse Links

 http://www.dwinfocenter.org
 http://www.datawarehouse.com
 http://www.datamining.org
 http://www.olapcouncil.org
 http://www.idwa.org
 http://www.knowledgecenters.org/dwcenter.asp

Recommended Perl Programming Links

 http://language.perl.com/style/

Please also read the DBI FAQ which is installed as a DBI::FAQ module. You can use perldoc to read it by executing the CWperldoc DBI::FAQ command.

DBI by Tim Bunce. This pod text by Tim Bunce, J. Douglas Dunlop, Jonathan Leffler and others. Perl by Larry Wall and the CWperl5-porters.

The DBI module is Copyright (c) 1994-2004 Tim Bunce. Ireland. All rights reserved.

You may distribute under the terms of either the GNU General Public License or the Artistic License, as specified in the Perl README file.

The DBI is free Open Source software. IT COMES WITHOUT WARRANTY OF ANY KIND.

My consulting company, Data Plan Services, offers annual and multi-annual support contracts for the DBI. These provide sustained support for DBI development, and sustained value for you in return. Contact me for details.

The DBI Roadmap is available at <http://search.cpan.org/~timb/DBI/Roadmap.pod>

If your company would benefit from a specific new DBI feature, please consider sponsoring its development. Work is performed rapidly, and usually on a fixed-price payment-on-delivery basis. Contact me for details.

Using such targeted financing allows you to contribute to DBI development, and rapidly get something specific and valuable in return.

I would like to acknowledge the valuable contributions of the many people I have worked with on the DBI project, especially in the early years (1992-1994). In no particular order: Kevin Stock, Buzz Moschetti, Kurt Andersen, Ted Lemon, William Hails, Garth Kennedy, Michael Peppler, Neil S. Briscoe, Jeff Urlwin, David J. Hughes, Jeff Stander, Forrest D Whitcher, Larry Wall, Jeff Fried, Roy Johnson, Paul Hudson, Georg Rehfeld, Steve Sizemore, Ron Pool, Jon Meek, Tom Christiansen, Steve Baumgarten, Randal Schwartz, and a whole lot more.

Then, of course, there are the poor souls who have struggled through untold and undocumented obstacles to actually implement DBI drivers. Among their ranks are Jochen Wiedmann, Alligator Descartes, Jonathan Leffler, Jeff Urlwin, Michael Peppler, Henrik Tougaard, Edwin Pratomo, Davide Migliavacca, Jan Pazdziora, Peter Haworth, Edmund Mergl, Steve Williams, Thomas Lowery, and Phlip Plumlee. Without them, the DBI would not be the practical reality it is today. I'm also especially grateful to Alligator Descartes for starting work on the first edition of the Programming the Perl DBI book and letting me jump on board.

The DBI and DBD::Oracle were originally developed while I was Technical Director (CTO) of the Paul Ingram Group (www.ig.co.uk). So I'd especially like to thank Paul for his generosity and vision in supporting this work for many years.

As you can see above, many people have contributed to the DBI and drivers in many ways over many years.

If you'd like to help then see <http://dbi.perl.org/contributing> and <http://search.cpan.org/~timb/DBI/Roadmap.pod>

If you'd like the DBI to do something new or different then a good way to make that happen is to do it yourself and send me a patch to the source code that shows the changes. (But read Speak before you patch below.)

Use http://svn.perl.org/modules/dbi/trunk (basic) or http://svn.perl.org/viewcvs/modules/ (more useful)

The DBI source code is maintained using Subversion (a replacement for CVS, see <http://subversion.tigris.org/>). To access the source you'll need to install a Subversion client. Then, to get the source code, do:

  svn checkout http://svn.perl.org/modules/dbi/trunk

If it prompts for a username and password use your perl.org account if you have one, else just 'guest' and 'guest'. The source code will be in a new subdirectory called CWtrunk.

To keep informed about changes to the source you can send an empty email to dbi-changes@perl.org after which you'll get an email with the change log message and diff of each change checked-in to the source.

After making your changes you can generate a patch file, but before you do, make sure your source is still upto date using:

  svn update

If you get any conflicts reported you'll need to fix them first. Then generate the patch file from within the CWtrunk directory using:

  svn diff > foo.patch

Read the patch file, as a sanity check, and then email it to dbi-dev@perl.org.

Unpack a fresh copy of the distribution:

  tar xfz DBI-1.40.tar.gz

Rename the newly created top level directory:

  mv DBI-1.40 DBI-1.40.your_foo

Edit the contents of DBI-1.40.your_foo/* till it does what you want.

Test your changes and then remove all temporary files:

  make test && make distclean

Go back to the directory you originally unpacked the distribution:

  cd ..

Unpack another copy of the original distribution you started with:

  tar xfz DBI-1.40.tar.gz

Then create a patch file by performing a recursive CWdiff on the two top level directories:

  diff -r -u DBI-1.40 DBI-1.40.your_foo > DBI-1.40.your_foo.patch

For anything non-trivial or possibly controversial it's a good idea to discuss (on dbi-dev@perl.org) the changes you propose before actually spending time working on them. Otherwise you run the risk of them being rejected because they don't fit into some larger plans you may not be aware of.

A German translation of this manual (possibly slightly out of date) is available, thanks to O'Reilly, at:

  http://www.oreilly.de/catalog/perldbiger/

Some other translations:

 http://cronopio.net/perl/                              - Spanish
 http://member.nifty.ne.jp/hippo2000/dbimemo.htm        - Japanese

References to DBI related training resources. No recommendation implied.

  http://www.treepax.co.uk/
  http://www.keller.com/dbweb/

(If you offer professional DBI related training services, please send me your details so I can add them here.)

See the DBI FAQ for a more comprehensive list of FAQs. Use the CWperldoc DBI::FAQ command to read it.

Read the information in the references below. Please do not post CGI related questions to the dbi-users mailing list (or to me).

 http://www.perl.com/cgi-bin/pace/pub/doc/FAQs/cgi/perl-cgi-faq.html
 http://www3.pair.com/webthing/docs/cgi/faqs/cgifaq.shtml
 http://www-genome.wi.mit.edu/WWW/faqs/www-security-faq.html
 http://www.boutell.com/faq/
 http://www.perl.com/perl/faq/

For information on the Apache httpd server and the CWmod_perl module see

  http://perl.apache.org/

Apache::DBI by E.Mergl@bawue.de

To be used with the Apache daemon together with an embedded Perl interpreter like CWmod_perl. Establishes a database connection which remains open for the lifetime of the HTTP daemon. This way the CGI connect and disconnect for every database access becomes superfluous.

JDBC Server by Stuart 'Zen' Bishop zen@bf.rmit.edu.au

The server is written in Perl. The client classes that talk to it are of course in Java. Thus, a Java applet or application will be able to comunicate via the JDBC API with any database that has a DBI driver installed. The URL used is in the form CWjdbc:dbi://host.domain.etc:999/Driver/DBName. It seems to be very similar to some commercial products, such as jdbcKona.

Remote Proxy DBD support

As of DBI 1.02, a complete implementation of a DBD::Proxy driver and the DBI::ProxyServer are part of the DBI distribution.

SQL Parser

See also the SQL::Statement module, SQL parser and engine.