Containment of Conjunctive Queries over Databases with Null Values

摘要

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.rnWe show that null-containment for boolean conjunctive queries is NP-complete while it is Π_2~p-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 Π_2~p-hard for boolean queries, while it remains in Π_2~p for arbitrary queries.