About Codd's theorem

Question

I was learning about databases and I have developed a rough idea that the design of databases has got quite a lot to do with mathematics. The most common and probably advanced form of databases are relational databases. The very beginning of its definition in standard textbooks say that it is a mathematical model devised by Codd. On searching I got to know that it is based on Codd's theorem which posits and provides a mathematical definition for the "expressive power . I would like to know which branch of mathematics does codd theorem belong to and what other areas of mathematics does databse design pertain to (or will possibly pertain to in future). Hope it's not a broad question to be closed . Eagerly waiting for reply , thanks in advance.