I would recommend SQL and Relational Theory by CJ Date. It completely changed how I use SQL. Before reading it, I had about a decade of experience with SQL. I

I would recommend SQL and Relational Theory by CJ Date. It completely changed how I use SQL. Before reading it, I had about a decade of experience with SQL. I didn't really understand the theory underneath. I also didn't understand how to write SQL in a way that adheres to the theory. You can think of this book as being the closest we have to "SQL: The Good Parts".