Universit a Degli Studi the Expressive Power of Stratiied Logic Programs with Value Invention Programs with Value Invention for Expressing Database Transformations" in International

The expressive power of the family wILOG(:) of relational query languages is investigated. The languages are rule based, with value invention and strati ed negation. The semantics for value invention is based on Skolem functor terms. We study a hierarchy of languages based on the number of strata allowed in programs. We rst show that, in presence of value invention, the class of strati ed programs made of two strata has the whole expressive power of the family, thus expressing the computable queries. We then show that the language wILOG 6= of programs with non-equality and without negation expresses the monotone queries, and that the language wILOG 12 ;: of semipositive programs expresses the semimonotone queries.