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.
Anoth...
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Author information
Authors and Affiliations
Editor information
Editors and Affiliations
Rights and permissions
Copyright information
© 2009 Springer Science+Business Media, LLC
About this entry
Cite this entry
Sirangelo, C. (2009). Join. In: LIU, L., ÖZSU, M.T. (eds) Encyclopedia of Database Systems. Springer, Boston, MA. https://doi.org/10.1007/978-0-387-39940-9_1260
Download citation
DOI: https://doi.org/10.1007/978-0-387-39940-9_1260
Publisher Name: Springer, Boston, MA
Print ISBN: 978-0-387-35544-3
Online ISBN: 978-0-387-39940-9
eBook Packages: Computer ScienceReference Module Computer Science and Engineering