Prolog allows unification, which doesn't have a parallel in relational algebra. I'm not sure how much more power this gives you without recursion, though.

If you're interested in connections between relational algebra (and SQL, and databases) and logic programming, you should look at Datalog, which is a restricted subset of Prolog that is akin to relational algebra plus fixed points (transitive closure, for example). In particular, Datalog forbids compound terms (like lists) and recursion in a negated position.

I think Datalog without recursion/fixed-points is precisely as powerful as relational algebra, but I don't have a proof handy.