Relational algebra is not Turing complete. SQL gets Turing completeness by means of various additions/alterations (sometimes pretty weird ones about that). So the foundations are pretty different, and I think this is obvious if you tried to start writing typical programs in SQL/Prolog.

Now, some people argue you might want a less expressive logic programming language, especially datalog, so this isn’t a knock-down argument, but it’s a pretty big difference.