Containment of conjunctive queries over databases with null values

  • Authors:

  • Carles Farré;Werner Nutt;Ernest Teniente;Toni Urpí

  • Affiliations:

  • Departament de Llenguatges i Sistemes Informatics, Unversitat Politècnica de Catalunya, Barcelona, Spain;Faculty of Computer Science, Free University of Bozen-Bolzano, Bozen, Italy;Departament de Llenguatges i Sistemes Informatics, Unversitat Politècnica de Catalunya, Barcelona, Spain;Departament de Llenguatges i Sistemes Informatics, Unversitat Politècnica de Catalunya, Barcelona, Spain

  • Venue:
  • ICDT'07 Proceedings of the 11th international conference on Database Theory
  • Year:
  • 2007

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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 $\mathit\Pi^{\rm P}_{2}$-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 $\mathit\Pi^{\rm P}_{2}$-hard for boolean queries, while it remains in $\mathit\Pi^{\rm P}_{2}$ for arbitrary queries.