# Encyclopedia of Database Systems

2018 Edition
| Editors: Ling Liu, M. Tamer Özsu

# Selection

• Cristina Sirangelo
Reference work entry
DOI: https://doi.org/10.1007/978-1-4614-8265-9_1257

## Synonyms

Selection (Relational Algebra)

## Definition

Given a relation instance R over set of attributes U and a condition F, the selection σF(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....

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