Synonyms
Definition
The union of two relation instances R 1 and R 2 over the same set of attributes U – denoted by R 1 ∪ R 2 – is another relation instance over U containing precisely the set of tuples t such that t ∈ R 1 or t ∈ R 2.
Key Points
The union is one of the primitive operators of the relational algebra. It is a natural extension of the set union to relations; the additional restriction is that it can be applied only to relations over the same set of attributes. However the union of two arbitrary relations having the same arity can be obtained by first renaming the attributes of one of the two relations.
As an example, consider a relation Students over attributes (number, name), containing tuples {(1001,Black), (1002, White)}, and a relation Employees over attributes (number, name), containing tuples {(1001,Black), (1003, Brown)}. Then the union Students ∪ Employees is a relation over attributes (number, name) with tuples {(1001, Black), (1002, White), (1003, Brown)}.
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© 2009 Springer Science+Business Media, LLC
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Sirangelo, C. (2009). Union. In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_1261
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DOI: https://doi.org/10.1007/978-0-387-39940-9_1261
Publisher Name: Springer, Boston, MA
Print ISBN: 978-0-387-35544-3
Online ISBN: 978-0-387-39940-9
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