Encyclopedia of Database Systems

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


  • Cristina SirangeloEmail author
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1261


The union of two relation instances R1 and R2 over the same set of attributes U – denoted by R1R2 – is another relation instance over U containing precisely the set of tuples t such that tR1 or tR2.

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 StudentsEmployees is a relation over attributes (number, name) with tuples {(1001, Black), (1002, White), (1003, Brown)}.

In the absence of...

This is a preview of subscription content, log in to check access.

Copyright information

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

Authors and Affiliations

  1. 1.IRIFParis Diderot UniversityParisFrance

Section editors and affiliations

  • Leonid Libkin
    • 1
  1. 1.School of InformaticsUniversity of EdinburghEdinburghUK