Selection (relational algebra)

In relational algebra, a selection is a unary operation written as σ_{a θ b}(R) or σ_{a θ v}(R) where:

a and b are attribute names
θ is a binary operation in the set {<, ≤, =, >, ≥}
v is a value constant
R is a relation

The selection σ_{a θ b}(R) selects all those tuples in R for which θ holds between the a and the b attribute.

The selection σ_{a θ v}(R) selects all those tuples in R for which θ holds between the a attribute and the value v.

For an example, consider the following tables where the first table gives the relation Person, the second table gives the result of σ_{Age ≥ 34}(Person) and the third table gives the result of σ_{Age = Weight}(Person).

Person

σ_{Age ≥ 34}(Person)

σ_{Age = Weight}(Person)

Name	Age	Weight
Harry	34	80
Sally	28	64
George	29	70
Helena	54	54
Peter	34	80

Name	Age	Weight
Harry	34	80
Helena	54	54
Peter	34	80

Name	Age	Weight
Helena	54	54

More formally the semantics of the selection is defined as follows:

σ_{a θ b}(R) = {t | t ∈ R, t(a) θ t(b)}
σ_{a θ v}(R) = {t | t ∈ R, t(a) θ v}

The result of the selection is only defined if the attribute names that it mentions are in the header of the relation that it operates upon.

In SQL, selections are performed by using WHERE definitions in SELECT, UPDATE, and DELETE statements.