[bitwize]

Understanding relational algebra helps you write better SQL the way understanding lambda calculus helps you write better JavaScript. The language is warty and but a shadow of the pristine purity underneath, but it gets the job done and the theory guides your reasoning on how to use the language.

[lkuty]

In particular, I find the closure property of SQL very helpful and always think in terms of queries gettings tables and producing tables (derived most of the time). https://en.wikipedia.org/wiki/Closure_(mathematics)