Preference rules in database querying

The paper proposes the use of preferences for querying databases. In expressing queries it is natural to express preferences among tuples belonging to the answer. This can be done in commercial DBMS, for instance, by ordering the tuples in the result. The paper presents a different proposal, based on similar approaches deeply investigated in the artificial intelligence field, where preferences are used to restrict the result of queries posed over databases. In our proposal a query over a database DB is a triple (q, T, Phi), where q denotes the output relation, P a Datalog program (or an SQL query) used to compute the result and Phi is a set of preference rules used to introduce preferences on the computed tuples. In our proposal tuples which are "dominated" by other tuples do not belong to the result and cannot be used to infer other tuples. A new stratified semantics is presented where the program T is partitioned into strata and the preference rules associated to each stratum of P are divided into layers; the result of a query is carried out by computing one stratum at time and by applying the preference rules, one layer at time. We show that our technique is sound and that the complexity of computing queries with preference rules is still polynomial.