Datalog as a Query Language for Data Exchange Systems

The class of unions of conjunctive queries (UCQ) has been shown to be particularly well-behaved for data exchange; its certain answers can be computed in polynomial time (in terms of data complexity). However, this is not the only class with this property; the certain answers to any

Datalog

program can also can be computed in polynomial time. The problem is that both UCQ and

Datalog

do not allow for negated atoms, while most database query languages are equipped with negation. Unfortunately, adding an unrestricted form of negation to these languages yields to intractability of the problem of computing certain answers.

In order to face this challenge, we have recently proposed a language, called

Datalog

C( ≠ )

[5], that extends

Datalog

with a restricted form of negation while keeping the good properties of

Datalog

, and UCQ, for data exchange. In this article, we provide evidence in favor of the use of

Datalog

C( ≠ )

as a query language for data exchange systems. More precisely, we introduce the syntax and semantics of

Datalog

C( ≠ )

, we present some of the fundamental results about this language shown in [5], and we extend those results to the case of data exchange settings that allow for constraints in the target schema. All of these results provide justification for the use of

Datalog

C( ≠ )

in practice.