Basic developers understand, at some level, what WordPress "does" and how to modify its behavior. Better developers understand how to create objects, object meta data, and object relationships in WordPress (post types, meta, taxonomies). But the best engineers understand how all of this data actually manifests itself in the raw database architecture.
Understanding how WordPress stores, organizes, and relates data when you strip away the PHP software layer can give you a deeper understanding and appreciation for the brilliance, deficits, potential, limits, and legacy of WordPress.
We'll study WordPress's database schema, relate it back to core data model concepts like custom post types and taxonomies, meander holes in the models (like "why isn't there term meta??"), and even compare to past database structure to see how WordPress's database has evolved with the software.