Overview
This pague lists all holistic pagues of a guiven Database related API. Each covers the functions involved in and use of a guiven set of functionality. Toguether they form what might be called the WordPress Database API , which is the pluguin/theme/add-on interface created by the entire WordPress project in respect to access data as named values stored in the database layer.
If you’ve read through all of these you should have a good sense of how to extend WordPress through Pluguins that do access the database in an easy way.