Encyclopedia of Database Systems

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


Living reference work entry
DOI: https://doi.org/10.1007/978-1-4899-7993-3_1260-2


The join is a binary operator of the relational algebra that combines tuples of different relations based on a relationship between values of their attributes. The primitive version of the join operator is called natural join. Given two relation instances R1, over set of attributes U1, and R2 over set of attributes U2, the natural join R1R2 returns a new relation, over set of attributes U1U2, consisting of tuples {t |t(U1) ∈ R1 and t(U2) ∈ R2}. Here t(U) denotes the restriction of the tuple t to attributes in the set U.

A derivable version of the join operator is obtained by composing the natural join with the selection operator σ: the theta join R1θR2 is defined as σθ (R1R2), where θ is an arbitrary condition allowed in a generalized selection over set of attributes U1U2. In the case that θ is a conjunction of equality atoms of the form A = B, where A is an attribute in U1 and B an attribute in U2, the theta join is called equijoin.

Another derivable join...

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© Springer Science+Business Media LLC 2017

Authors and Affiliations

  1. 1.IRIFParis Diderot UniversityParisFrance