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This page lists all holistic pages of a given Database related API. Each covers the functions involved in and use of a given set of functionality. Together they form what might be called the WordPress Database API, which is the plugin/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 Plugins that do access the database in an easy way.