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Describe the following Relational Algebra operations
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(i)Select:

• The select operator is used to select the rows from a table which satisfy particular selection condition given in selection operation.

• Select operator selects a set of tuples that satisfy a selection condition.

• Output of query is exactly same as input schema of table.

• This is unary relational operator having only one input table.

Syntax:

σ<attribute_name><comparison_operator><constant_value>(<input_table_name>)

Where,

Attribute_name: Name of the column in table

Comparison_operator: =,<,<=,>,>=,<>

Example:

Eid EName Age
1 Nidhi 34
2 Ajit 24
3 Amit 28

Select all the employees having age below 25 years

     σage<25(Employee)
Eid EName Age
2 Ajit 24

ii) Project

• Project operator is used foe selecting some of many columns in table to display in result set.

• Few rows and columns can be selected as per requirement.

• This unary relational operator having only one input table.

Syntax:

         π<column_list>(<input_table_name>)

Example:

Find Employees salary from Employee table and Salary

         πEid,Salary,Age(Employee)
Eid Salary Age
1 100000 34
2 12000 24
3 24000 28

ii) Natural Join

• A Natural join returns all rows by matching values in comman columns having same name and data type of columns should be presentin both sides

Example:

Find all the Employees and their department name

Employee Table

Eid EName Did
1 Amit 10
2 Nidhi 30
3 Ajit 50

Department Table

Did DName
10 IPF
30 HR
70 TIS
        πeid,dname(Employee ⋈ Department)
Eid DName
1 IPF
2 HR

iv) Set Intersection

• Set Intersection operator finds out all rows that are common in both result of Query 1 and in the result of Query 2.

Syntax: (Query Expression 1) ⋂ (Query Expression 2)

R

A 1
B 2
C 3
D 4
E 5

S

A 1
B 2
C 3
D 4

R ⋂S

A 1
B 2
C 3
D 4

Examples:

All Employees in IT Department under Mumbai University

IT_Employee

Eid EName Age
11 Suhas 24
12 Jayendra 24
13 Sachin 25
14 Mahesh 23

All Employess in ABC College

Eid EName Age
11 Suhas 24
12 Jayendra 24
23 Geeta 25
24 Amruta 23
35 Sangeeta 21

Intersection of the above is employees in IT Department of ABC College

Eid EName Age
11 Suhas 24
12 Jayendra 24
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