Synonyms
Projection (Relational Algebra)
Definition
Given a relation instance R over set of attributes U, and given a subset X of U, the projection of R on X – denoted by πX(R) – is defined as a relation over set of attributes X whose tuples are the restriction of tuples of R to attributes X. That is, t ∈ πX(R) if and only if t = t′(X) for some tuple t′ of R (here t′(X) denotes the restriction of t′ to attributes X).
Key Points
The projection is one of the basic operators of the relational algebra. It operates by “restricting” the input relation to some of its columns.
The arity of the output relation is bounded by the arity of the input relation. Moreover the number of tuples in πX(R) is bounded by the number of tuples in R. In particular, the size of πX(R) can be strictly smaller than the size of R since different tuples of R may have the same values on attributes X.
As an example, consider a relation Goods over attributes (code, price, quantity), containing tuples {(001, 5.00, 10),...
This is a preview of subscription content, log in via an institution.
Buying options
Tax calculation will be finalised at checkout
Purchases are for personal use only
Learn about institutional subscriptionsAuthor 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). Projection. 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_1258
Download citation
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1258
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