Encyclopedia of Database Systems

2018 Edition
| Editors: Ling Liu, M. Tamer Özsu

Expressive Power of Query Languages

  • Leonid Libkin
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1239

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

     
  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, 12 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...

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Recommended Reading

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Copyright information

© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.School of InformaticsUniversity of EdinburghEdinburghUK

Section editors and affiliations

  • Leonid Libkin
    • 1
  1. 1.School of InformaticsUniversity of EdinburghEdinburghUK