In this excerpt from the Overview of Drupal 7, Angie Byron, talks about the Database API in Drupal 7. This was a round table discussion with other Lullabots on the line who ask and answer questions, in addition to Angie's presentation. This lays a good foundation for how the database system works in Drupal and sets you up for the hands-on tutorials that follow.
Check your version
This video covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.