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## 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.
• It is a fundamental / Basic operator
• It is a unary operator

 π(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

### Result for Query πName, Age(Student)-

 Name Age Ashish 19 Rahul 20 Naina 20 Sameer 21

### Result for Query πID , Name(Student)-

 ID Name 100 Ashish 200 Rahul 300 Naina 400 Sameer

## 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:

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

## Point-06:

• 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.