Encyclopedia of Database Systems

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

Difference

  • Cristina Sirangelo
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1262

Synonyms

Set difference

Definition

The difference of two relation instances R1 and R2 over the same set of attributes U, denoted by R1 − R2, is another relation instance over U containing precisely the set of tuples occurring in R1 and not occurring in R2.

Key Points

The difference is one of the primitive operators of the relational algebra. It coincides with the notion of set difference, with the additional restriction that it can be applied only to relations over the same set of attributes. However the difference of two arbitrary relations having the same arity can be obtained by first renaming attributes in 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 difference Students − Employees is a relation over attributes (number, name) containing the only tuple (1002, White)....

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