Containment of Conjunctive Queries over Databases with Null Values

  • Carles Farré
  • Werner Nutt
  • Ernest Teniente
  • Toni Urpí
Conference paper
Part of the Lecture Notes in Computer Science book series (LNCS, volume 4353)


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.


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Copyright information

© Springer-Verlag Berlin Heidelberg 2006

Authors and Affiliations

  • Carles Farré
    • 1
  • Werner Nutt
    • 2
  • Ernest Teniente
    • 1
  • Toni Urpí
    • 1
  1. 1.Departament de Llenguatges i Sistemes InformaticsUnversitat Politècnica de CatalunyaBarcelonaSpain
  2. 2.Faculty of Computer ScienceFree University of Bozen-BolzanoBozenItaly

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