Selection Operator in Relational Algebra

 

Selection Operator-

 

·   Selection Operator (σ) is a unary operator in relational algebra that performs a selection operation.

·   It selects those rows or tuples from the relation that satisfies the selection condition.

·   It is a fundamentals / Basic operator

·   It is a Unary Operator

 

σ_{<selection_condition>}(R)

 

·   Select tuples from a relation “Books” where subject is “database”

σ_{subject = “database”} (Books)

·   Select tuples from a relation “Books” where subject is “database” and price is “450”

σ_{subject = “database” ∧ price = “450”} (Books)

·   Select tuples from a relation “Books” where subject is “database” and price is “450” or have a publication year after 2010

σ_{subject = “database” ∧ price = “450” ∨ year >”2010″} (Books)

Important Points-

 

Point-01:

 

·   We may use logical operators like ∧ , ∨ , ! and relational operators like  = , ≠ , > , < , <= , >= with the selection condition.

Point-02:

 

·   Selection operator only selects the required tuples according to the selection condition.

·   It does not display the selected tuples.

·   To display the selected tuples, projection operator is used.

Point-03:

 

·   Selection operator always selects the entire tuple. It can not select a section or part of a tuple.

Point-04:

 

·   Selection operator is commutative in nature i.e.

σ _{A ∧ B} (R) = σ _{B ∧ A} (R)

OR

σ _B (σ _A(R)) = σ _A (σ _B(R))

 

Point-05:

 

·   Degree of the relation from a selection operation is same as degree of the input relation.

Point-06:

 ·        The number of rows returned by a selection operation is obviously less than or equal to the number of rows in the original table.

·        Minimum Cardinality = 0

·        Maximum Cardinality = |R|