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.
This work was supported by the British EPSRC (Grant GR/SS44830/01 MAGIK-I) and the Spanish Ministerio de Educacion y Ciencia (Grant TIN 2005-05406).
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Farré, C., Nutt, W., Teniente, E., Urpí, T. (2006). Containment of Conjunctive Queries over Databases with Null Values. In: Schwentick, T., Suciu, D. (eds) Database Theory – ICDT 2007. ICDT 2007. Lecture Notes in Computer Science, vol 4353. Springer, Berlin, Heidelberg. https://doi.org/10.1007/11965893_27
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DOI: https://doi.org/10.1007/11965893_27
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