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.

 

Syntax-

 

σ<selection_condition>(R)

 

Examples-

 

• 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.

Thus,

• Minimum Cardinality = 0
• Maximum Cardinality = |R|

 

Next Article- Projection Operation in Relational Algebra

 

Get more notes and other study material of Database Management System (DBMS).

Watch video lectures by visiting our YouTube channel LearnVidFun.