On the containment of conjunctive queries

Abstract

Testing containment of conjunctive queries is the question whether for any such queries Q1,Q2, for any database instance D, the set of answers of one query is contained in the set of answers of the other. In this paper, we first introduce into the general approach for testing for the case, when both queries do not contain negation. Based on these techniques we discuss the question for the case of queries with negation. We propose a new method and compare with the currently known approach. Instead of always testing the exponential number of possible canonical databases, our algorithm will terminate once a certain canonical database is constructed. However, in the worst case, still an exponential number of canonical databases has to be checked. © Springer-Verlag Berlin Heidelberg 2003.