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 R1, over set of attributes U1, and R2 over set of attributes U2, the natural join R1 ⋈ R2 returns a new relation, over set of attributes U1 ∪ U2, 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 R1 ⋈θR2 is defined as σθ (R1 ⋈ R2), where θ is an arbitrary condition allowed in a generalized selection over set of attributes U1 ∪ U2. 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...
Access this chapter
Tax calculation will be finalised at checkout
Purchases are for personal use only
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Section Editor information
Rights and permissions
Copyright information
© 2018 Springer Science+Business Media, LLC, part of Springer Nature
About this entry
Cite this entry
Sirangelo, C. (2018). Join. In: Liu, L., Özsu, M.T. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4614-8265-9_1260
Download citation
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1260
Published:
Publisher Name: Springer, New York, NY
Print ISBN: 978-1-4614-8266-6
Online ISBN: 978-1-4614-8265-9
eBook Packages: Computer ScienceReference Module Computer Science and Engineering