DEFAULT LOGIC AS A QUERY LANGUAGE

Research in nonmonotonic reasoning has focused largely on the idea of representing knowledge about the world via rules that are generally tr ue but can be defeated. Even if relational databases are nowadays the main tool for storing very large sets of data, the approach of using n onmonotonic Al formalisms as relational database query languages has b een investigated to a much smaller extent. In this work, we propose a novel application of Reiter's default logic by introducing a default q uery language (DQL) for finite relational databases, which is based on default rules. The main result of this paper is that DQL is as expres sive as SO There Exists For All, the existential-universal fragment of second-order logic. This result is not only of theoretical importance : We exhibit queries-which are useful in practice--that can be express ed with DQL and cannot with other query languages based on nonmonotoni c logics such as DATALOG with negation under the stable model semantic s. In particular, we show that DQL is well-suited for diagnostic reaso ning.