Encyclopedia of Database Systems

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

Conjunctive Query

  • Val Tannen
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1002

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. 1.
    Abiteboul S, Hull R, Vianu V. Foundations of databases: the logical level. Reading: Addison Wesley; 1994.Google Scholar
  2. 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.Google Scholar
  3. 3.
    Ullman JD. Principles of database and knowledge-base systems, vol. I. Rockville: Computer Science Press; 1988.Google Scholar

Copyright information

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

Authors and Affiliations

  1. 1.Department of Computer and Information ScienceUniversity of PennsylvaniaPhiladelphiaUSA

Section editors and affiliations

  • Val Tannen
    • 1
  1. 1.Dept. of Computer and Inf. ScienceUniv. of PennsylvaniaPhiladelphiaUSA