Datalog also lets you define facts and rules, in addition to posting queries. A key attraction of Datalog is that the same simple and very uniform syntax is use

Datalog also lets you define facts and rules, in addition to posting queries. A key attraction of Datalog is that the same simple and very uniform syntax is used in all cases. All these cases are special cases of a single unifying language element called Horn clauses, which are clauses with at most one positive literal: Queries have no positive literal, rules have exactly one positive literal and at least one negative literal, and facts consist of a single positive literal.