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Encyclopedia of Database Systems pp 1–4Cite as

Expressive Power of Query Languages

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Definition

The study of expressive power concentrates on comparing classes of queries that can be expressed in different languages, and on proving expressibility - or inexpressibility - of certain queries in a query language.

Historical Background

Ever since Codd proposed relational calculus (first-order predicate logic) as a basic relational query language, it has been common for database query languages to have limited expressiveness. If a language cannot express everything computable, then it is natural to ask:

  1. 1.

    What queries cannot be expressed in a language ℒ

  2. 2.

    Which methods are available for proving such results?

Furthermore, if there are two query languages ℒ 1 and ℒ 2, one may want to compare their expressiveness: for example, ℒ 1 ⊈ ℒ 2 means that all queries expressible in ℒ 1 are also expressible in ℒ 2, but there are queries expressible in ℒ 2 that are not expressible in ℒ 1.

In 1975, Fagin [4] showed that queries such as the transitive closure of a graph and connectivity...

Keywords

  • Query Language
  • Transitive Closure
  • Aggregate Function
  • Parity Query
  • Datalog Program

These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Author information

Authors and Affiliations

  1. School of Informatics, University of Edinburgh, Crichton Street, EH8 9LE, Edinburgh, Scotland, UK

    Leonid Libkin

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  1. Leonid Libkin
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Correspondence to Leonid Libkin .

Editor information

Editors and Affiliations

  1. Georgia Institute of Technology College of Computing, Atlanta, Georgia, USA

    Ling Liu

  2. University of Waterloo School of Computer Science, Waterloo, Ontario, Canada

    M. Tamer Özsu

Section Editor information

  1. School of Informatics, University of Edinburgh, Crichton Street, EH8 9LE, Edinburgh, Scotland, UK

    Leonid Libkin

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Libkin, L. (2016). Expressive Power of Query Languages. In: Liu, L., Özsu, M. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4899-7993-3_1239-2

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  • DOI: https://doi.org/10.1007/978-1-4899-7993-3_1239-2

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