Encyclopedia of Database Systems

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


  • David W. EmbleyEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_811


Uniqueness constraint


In the relational model, a key for a relational schema is a set of attributes whose value(s) uniquely identify a tuple in a valid instance of the relation. Said another way, key value(s) appear at most once in a relation. Often the set of attributes constituting a key is a set with a single attribute. For example, in the relation
$$ \mathrm{Customer} \frac{\left(\mathrm{Customer}\; \mathrm{ID}\quad \mathrm{Name}\quad \mathrm{Address}\right)} {\begin{array}{l} 11111\qquad \qquad \mathrm{Pat}\qquad 12\ \mathrm{Maple}\\ {}22222\qquad \qquad \mathrm{Tracy}\qquad 44\ \mathrm{Elm} \end{array}} $$
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© Springer Science+Business Media, LLC, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Brigham Young UniversityProvoUSA

Section editors and affiliations

  • David W. Embley
    • 1
  1. 1.Brigham Young UniversityProvoUSA