You have full access to this open access chapter, Download reference work entry PDF
Keywords
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Synonyms
Definition
Given a relation instance R over set of attributes U and a condition F, the selection σ F (R) returns a new relation over U consisting of the set of tuples of R which satisfy F. The condition F is an atom of the form A = B or A = c, where A and B are attributes in U and c is a constant value.
The generalized selection allows more complex conditions: F can be an arbitrary Boolean combination of atoms of the form A = B or A ≠ B or A = c or A ≠ c. Moreover, if a total order is defined on the domain of attributes, more general comparison atoms of the form A α B or A α c are allowed, where α ranges over {=, ≠, <, >, ≤, ≥}.
Key Points
The selection is one of the basic operators of the relational algebra. It operates by “selecting” rows of the input relation. A tuple t over U satisfies the condition A = B if the values of attributes A and B in t are equal. Similarly t satisfies the condition A = c if the value of attribute A in t is c. Satisfaction of generalized selection atoms is defined analogously.
As an example, consider a relation Exams over attributes (course-number, student-number, grade), containing tuples {(EH1, 1001, A), (EH1, 1002, A), (GH5, 1001, C)}. Then σ grade=A∧course−number = EH1(Exams) is a relation over attributes (course-number, student-number, grade) with tuples {(EH1, 1001, A), (EH1, 1002, A)}.
In the case that a relation schema is only specified by a relation name and arity, the result of the selection is a new relation having the same arity as the input one, containing the tuples which satisfy the selection condition. In this case the selection atoms are expressions of the form j = k or j = c (or j α k and j α c in the generalized selection). Here j and k are positive integers bounded by the arity of the input relation, identifying its j-th and k-th attributes, respectively.
Cross-References
Author information
Authors and Affiliations
Corresponding author
Editor information
Editors and Affiliations
Section Editor information
Rights and permissions
Copyright information
© 2017 Springer Science+Business Media LLC
About this entry
Cite this entry
Sirangelo, C. (2017). Selection. In: Liu, L., Özsu, M. (eds) Encyclopedia of Database Systems. Springer, New York, NY. https://doi.org/10.1007/978-1-4899-7993-3_1257-2
Download citation
DOI: https://doi.org/10.1007/978-1-4899-7993-3_1257-2
Received:
Accepted:
Published:
Publisher Name: Springer, New York, NY
Print ISBN: 978-1-4899-7993-3
Online ISBN: 978-1-4899-7993-3
eBook Packages: Springer Reference Computer SciencesReference Module Computer Science and Engineering