Tag: Selection Operation in DBMS

Selection Operator | Relational Algebra | DBMS

Relational Algebra Operators-


Before you go through this article, make sure that you have gone through the previous article on Introduction to Relational Algebra.


The operators in relational algebra are classified as-



In this article, we will discuss about Selection Operator.


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.








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

σsubject = “database” (Books)

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

σsubject = “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

σsubject = “database” ∧ price = “450” ∨ year >”2010″ (Books)


Important Points-




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




  • 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.




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




  • Selection operator is commutative in nature i.e.

σ A ∧ B (R) = σ B ∧ A (R)


σ (σ A(R)) = σ (σ B(R))




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




  • 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|


Next Article- Projection Operation in Relational Algebra


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