Tag: Select Operation in DBMS

Selection Operator | Relational Algebra | DBMS

Database Management System

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”

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-

 

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

 

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