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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
the singleton set consisting of just CustomerID is a key and so is the set consisting of the pair of attributes (Name, Address). The values of CustomerID (here, 11111 and 22222) each uniquely identify a tuple. In any valid instance for this relation, no CustomerID value may appear twice. Similarly, the pair (Name, Address) is a key. Pairs of (Name, Address) values (e.g., <Pat, 12 Maple>) uniquely identify...
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Embley, D.W. (2017). Key. In: Liu, L., Özsu, M. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4899-7993-3_811-2
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DOI: https://doi.org/10.1007/978-1-4899-7993-3_811-2
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Publisher Name: Springer, New York, NY
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Online ISBN: 978-1-4899-7993-3
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