Containment of conjunctive queries over databases with null values

Abstract

We study containment of conjunctive queries that are evaluated over databases that may contain tuples with null values. We assume the semantics of SQL for single block queries with a SELECT DISTINCT clause. This problem ("null containment" for short) is different from containment over databases without null values and sometimes more difficult. We show that null-containment for boolean conjunctive queries is NP-complete while it is i72-complete for queries with distinguished variables. However, if no relation symbol is allowed to appear more than twice, then null-containment is polynomial, as it is for databases without nulls. If we add a unary test predicate IS NULL, as it is available in SQL, then containment becomes Π2-hard for boolean queries, while it remains in Π2 for arbitrary queries. © Springer-Verlag Berlin Heidelberg 2007.