In many respects "Datalog" doesn't refer to a single language or implementation or standard. It really just refers to a set of approaches for querying relational datasets using something like Prolog's unification.

By which I mean there are Datalogs that look like Prolog a bit, and others that don't. And things that are "Datalogs" that don't even have their own PL but instead more of an API. And no standard at all.