database theory book recommendations?

[Toby Douglass]

I'm currently applying for a developer job with mySQL.

I want to improve my database theory knowledge.

Any book recommendations?

Searching on-line is surprisingly unhelpful; the only things I've found so
far which looks half-sensible are "Database is Depth" by Chris Date, and
"Expert mySQL" by Charles A. Bell (which looks more practical rather than
theoretical).