You may not have caught on to what DBI really is. It's a uniform interface to any number of database specific modules. There is one DBI, and many DBD modules. There is even a template DBD called DBI::DBD, in case you come across a database for which a DBD doesn't exist, and you'd like to be able to access it through DBI.

If you look at the DBI documentation, the Description section starts with the following:

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.

Even before that, there's the following suggestion:

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 perldoc DBI::FAQ. However the DBI::FAQ module is currently (2004) outdated relative to the online FAQ on the DBI home page.

Clear?