I strongly recommend the book "Seven Databases in Seven Weeks" if you want to understand a bit of "theory" around databases. The first chapter focuses on a rel

I strongly recommend the book "Seven Databases in Seven Weeks" if you want to understand a bit of "theory" around databases. The first chapter focuses on a relational database, PostgreSQL, but by learning about the other types (columnar, document-store, graph, etc) you also learn more about relational databases by learning what choices they make and how they could have gone otherwise.