Drupal depends on a database to function correctly. Inside Drupal, a lightweight database abstraction layer exists between your code and the database. In this chapter, you’ll learn about how the database abstraction layer works and how to use it. You’ll see how queries can be modified by modules. Then, you’ll look at how to connect to additional databases (such as a legacy database). Finally, you’ll examine how the queries necessary to create and update database tables can be included in your module’s .install file by using Drupal’s schema API.