Encyclopedia of Database Systems

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

Key

Living reference work entry
DOI: https://doi.org/10.1007/978-1-4899-7993-3_811-2

Definition

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
$$\mathrm{Customer} \frac{\left(\mathrm{Customer} \mathrm{ID} \mathrm{Name} \mathrm{Address}\right)} {\begin{array}{l} 11111 \mathrm{Pat} 12\ \mathrm{Maple}\\ {}22222 \mathrm{Tracy} 44\ \mathrm{Elm} \end{array}}$$
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