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-

 

ID	Name	Subject	Age
100	Ashish	Maths	19
200	Rahul	Science	20
300	Naina	Physics	20
400	Sameer	Chemistry	21

Student

 

Then, we have-

 

Result for Query π_{Name, Age}(Student)-

 

Name	Age
Ashish	19
Rahul	20
Naina	20
Sameer	21

 

Result for Query π_{ID , Name}(Student)-

 

ID	Name
100	Ashish
200	Rahul
300	Naina
400	Sameer

 

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

 

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

Watch video lectures by visiting our YouTube channel LearnVidFun.