Drupal 7 Module Development

Database Access
Although Drupal 7 has made major leaps forward in terms of the flexibility of its
data storage in practice the vast majority of Drupal sites still rely on an SQL database
for both their primary data and for most of the configuration. In the past Drupal has
relied on a very thin database layer that provided only limited abstraction beyond
PHP's native support for the MySQL and PostgreSQL databases, which was a serious
limitation for complex or traffic-heavy sites.
Drupal 7, however, features a brand new database layer rewritten from the
ground up to provide a wide range of robust features in the areas of security,
scalability, and developer flexibility. Known somewhat tongue-in-cheek as
"Databases: The Next Generation" or "DBTNG" during development, it offers
support for many advanced SQL features as well as vastly improved portability
between the leading SQL databases on the market. In fact, Drupal 7 now ships with
support for the three leading open source databases (MySQL and variants such as
Maria DB, PostgreSQL, and SQLite) out-of-the-box and as of this writing add-on
drivers are available for both, Microsoft SQL Server and Oracle.
The database API is well documented, but this section will provide an overview of
the major features of the database API and how to use them to ensure fast, robust
code. We'll assume an existing knowledge of SQL. For more detailed information see
http://drupal.org/developing/api/database and http://api.drupal.org/
api/group/database/7.