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Expressive Power of Query Languages

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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

  2. 2.

    Which methods are available for proving such results?

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

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


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

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Correspondence to 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.

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