It took me a few years to truly grasp MySQL and the concept of relational databases. The MySQL Pocket Reference is the book that helped it all click for me.
I loved how small this was. I literally carried it around in my back pocket to school and work for a few weeks. I could skim the entire book in one sitting and it was a great way to drill the concepts in.
My favorite part is that this 15 year old book probably isn’t very dated since SQL has been SQL since the 60s.