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 (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.
Consider the following Student relation-
Then, we have-
Result for Query πName, Age(Student)-
Result for Query πID , Name(Student)-
- The degree of output relation (number of columns present) is equal to the number of attributes mentioned in the attribute list.
- 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.
- 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.
- Projection operator does not obey commutative property i.e.
π <list2> (π <list1> (R)) ≠ π <list1> (π <list2> (R))
- Following expressions are equivalent because both finally projects columns of list-1
π <list1> (π <list2> (R)) = π <list1> (R)
- Selection Operator performs horizontal partitioning of the relation.
- Projection operator performs vertical partitioning of the relation.
- 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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