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Encyclopedia of Database Systems pp 580–581Cite as

Conjunctive Query

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Synonyms

Horn clause query; SPC query

Definition

Conjunctive queries are first-order queries that both practically expressive and algorithmically relatively tractable. They were studied first in [2] and they have played an important role in database systems since then.

As a subset of the relational calculus, conjunctive queries are defined by formulae that make only use of atoms, conjunction, and existential quantification. As such they are closely related to Horn clauses and hence to logic programming. A single Datalog rule can be seen as a conjunctive query [1].

Optimization and reformulation for various purposes is quite feasible for conjunctive queries, as opposed to general relational calculus/algebra queries. The equivalence (and indeed the containment) of conjunctive queries is decidable, albeit NP-complete [1].

Key Points

This entry uses terminology defined in the entry Relational Calculus.

Conjunctive queries are first-order queries of a particular form: {〈e1, … , en〉 ∣ ∃ x1...

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Recommended Reading

  1. Abiteboul S, Hull R, Vianu V. Foundations of databases: the logical level. Reading: Addison Wesley; 1994.

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  2. Chandra AK, Merlin PM. Optimal implementation of conjunctive queries in relational data bases. In Proceedings of the 9th Annual ACM Symposium on Theory of Computing; 1977. p. 77–90.

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  3. Ullman JD. Principles of database and knowledge-base systems, vol. I. Rockville: Computer Science Press; 1988.

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Author information

Authors and Affiliations

  1. Department of Computer and Information Science, University of Pennsylvania, Philadelphia, PA, USA

    Val Tannen

Authors
  1. Val Tannen
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Corresponding author

Correspondence to Val Tannen .

Editor information

Editors and Affiliations

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

    Ling Liu

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

    M. Tamer Özsu

Section Editor information

  1. Dept. of Computer and Inf. Science, Univ. of Pennsylvania, Philadelphia, PA, USA

    Val Tannen

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Tannen, V. (2018). Conjunctive Query. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_1002

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