I'm not too well-versed in the academic/mathematical background of SQL, other than than to say it's closer to relational algebra than Set theory, and the resource to read is probably the 1970 Codd paper: https://www.seas.upenn.edu/~zives/03f/cis550/codd.pdf

SQL does implement the set operations of UNION, INTERSECT, and EXCEPT, but I meant "thinking in sets" more colloquially.

edit: this is more practical than theoretical but the author here actually does a nice job of discussing "set-based" thinking vs. procedural https://www.itprotoday.com/open-source-sql/fangraphs-tags-cl...

Discrete Mathematics with Applications by Epp. Chapters 5, 7, 10.

Fundamentals of Database Systems by Elmasri & Navathe. Part 2.