Wikipedia says that “Relational calculus is essentially equivalent to first-order logic, and indeed, Codd’s Theorem had been known to logicians since the late 1940s.” I couldn’t find the cited sources online, but did find these interesting papers:

“Applications of Alfred Tarski’s Ideas in Database Theory”

and

“Tarski’s influence on computer science.” (see the section starting “The final thing I want to tell something about is the connection of Tarski’s ideas and work with database theory.”)

If you’ve studied mathematical logic (or math, eg, topology), you are probably familiar with Tarski’s name. The historical development of mathematical logic and relational database theory is an interesting topic I’d like to understand better. Codd’s 1970 paper is fairly readable, and this 1972 paper is related to the correspondences between the different approaches taken.