[gregjor]

Query parsing, optimization, execution plans have to do with the SQL language. Relational databases (and some non-relational databases) use the SQL language but that language has little to do with the fundamentals of databases. The relational model has almost nothing to do with the query language, in other words.

I recommend Chris (C. J.) Dates books on databases. Introduction to Database Systems for a start.

Databases, especially relational databases, sit at the core of most business and web applications. Learning how to design, implement, and use databases effectively gives one a skill that will pay off for a long time, an entire career for some people.