Comment by j-pb

Most database literature simply uses Datalog to mean the query language fragment of conjunctive queries + recursion/fixpoint-iteration and potentially stratified negation.

Yes it started out as a Prolog subset, but the definition as the fragments it supports has become much more prevalent, mainly to contrast it to non-recursive fragments with arbitrary negation (e.g. SQL).

This usage dates back to database literature of the 80s by Ullman et. al.