The set theory? Look up relational algebra. These discussions get murky because people always conflate SQL the language with the underlying relational algebra, so it’s near impossible to criticize the language alone — like exactly in this case. It also gets conflated with the RDBMS engine, which doesn’t help either.

The article specifically calls out issues with the language, and specifically seeks to maintain the algebra with a different language… and somehow we’re back to talking about how sql is great because the algebra is great.

Codd created something beautiful; ibm created something significantly less so.