Projection 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 Projection Operator.

 

Projection Operator-

 

  • Projection Operator (π) is a unary operator in relational algebra that performs a projection operation.
  • It displays the columns of a relation or table based on the specified attributes.

 

Syntax-

 

π<attribute list>(R)

 

Example-

 

Consider the following Student relation-

 

IDNameSubjectAge
100AshishMaths19
200RahulScience20
300NainaPhysics20
400SameerChemistry21

Student

 

Then, we have-

 

Result for Query πName, Age(Student)-

 

NameAge
Ashish19
Rahul20
Naina20
Sameer21

 

Result for Query πID , Name(Student)-

 

IDName
100Ashish
200Rahul
300Naina
400Sameer

 

Important Points-

 

Point-01:

 

  • The degree of output relation (number of columns present) is equal to the number of attributes mentioned in the attribute list.

 

Point-02:

 

  • Projection operator automatically removes all the duplicates while projecting the output relation.
  • So, cardinality of the original relation and output relation may or may not be same.
  • If there are no duplicates in the original relation, then the cardinality will remain same otherwise it will surely reduce.

 

Point-03:

 

  • If attribute list is a super key on relation R, then we will always get the same number of tuples in the output relation.
  • This is because then there will be no duplicates to filter.

 

Point-04:

 

  • Projection operator does not obey commutative property i.e.

π <list2> (π <list1> (R)) ≠ π <list1> (π <list2> (R))

 

Point-05:

 

  • Following expressions are equivalent because both finally projects columns of list-1

π <list1> (π <list2> (R)) = π <list1> (R)

 

Point-06:

 

  • Selection Operator performs horizontal partitioning of the relation.
  • Projection operator performs vertical partitioning of the relation.

 

Point-07:

 

  • There is only one difference between projection operator of relational algebra and SELECT operation of SQL.
  • Projection operator does not allow duplicates while SELECT operation allows duplicates.
  • To avoid duplicates in SQL, we use “distinct” keyword and write SELECT distinct.
  • Thus, projection operator of relational algebra is equivalent to SELECT operation of SQL.

 

Next Article- Set Theory Operators in Relational Algebra

 

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Summary
Projection Operator | Relational Algebra | DBMS
Article Name
Projection Operator | Relational Algebra | DBMS
Description
Relational Algebra Operators- Projection Operator (π) is a unary operator in relational algebra that perform a projection operation. It displays the columns of a table or relation based on the specified attributes.
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Gate Vidyalay
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