Definition
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 R 1, over set of attributes U 1, and R 2 over set of attributes U 2, the natural join R 1 ⋈ R 2 returns a new relation, over set of attributes U 1 ∪ U 2, consisting of tuples {t |t(U 1) ∈ R 1 and t(U 2) ∈ R 2}. 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 R 1 ⋈ θ R 2 is defined as σ θ (R 1 ⋈ R 2), where θ is an arbitrary condition allowed in a generalized selection over set of attributes U 1 ∪ U 2. In the case that θ is a conjunction of equality atoms of the form A = B, where A is an attribute in U 1 and B an attribute in U 2, the theta join is called equijoin.
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Sirangelo, C. (2017). Join. In: Liu, L., Özsu, M. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4899-7993-3_1260-2
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DOI: https://doi.org/10.1007/978-1-4899-7993-3_1260-2
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