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Here we will have a discussion on Introduction to Relational Algebra and its operators in DBMS.
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Relational Algebra and its operators

Relational Algebra: Relational Database Management Systems are to be provided with a query language that helps its user to query the database instance.

Relational Algebra is a procedural query language that takes an instance of relation as the input and yields instance of relation as an output. Relational Algebra uses operators to perform queries.

To understand clearly the statements on relational algebra, consider a database management system with the ability to store the data but unable to fetch the data from the database.

It is of no use if the Database Management System is not provided with the ability to fetch the data.

In our previous section, we have discussed about SQL.

SQL is a software of the DBMS which provides all the features required to CREATE, SELECT, INSERT, DELETE and Many more operation to be done on databases.

Relational algebra is the mathematical background based on which the SQL has been designed.

Basic Fundamental Operators in Relational Algebra:

Operator                                                     

Select                                                                     
Project                                                                   
Rename                                                     

Union                                                                   
Intersection                                                        
Set Difference                                                    
Cartesian Product                                         
In this discussion, we will understand Select and Project operators.

In our next discussion, we will discuss the remaining relational algebra operators.

Consider the following tables to understand the operators.

SELECT () Operator: Select operation is used to choose a subset of tuples from a relation that satisfies the select conditions.

Select Operation is horizontal partition.

Consider the following queries.

(Dno = 6) EMP

What does the relational algebraic expression derive?

It is going to SELECT the subset of the rows for which the Selection condition Dno = 6 is true from the EMP table.

The SQL Query for the above Relational Algebraic Expression is " SELECT * FROM emp WHERE Dno = 6; ".
(Dnum = 5) Dept

What does the relational algebraic expression derive?

It is going to SELECT the subset of the rows for which the Selection condition Dnum = 5 is true from the DEPT table.
Similarly, we will consider more complex queries.

(Dno = 5 AND Sal 5000) OR (Dno = 6 AND Sal 20000) EMP
PROJECT () Operator: Project Operator SELECT specific columns from the table. The project operator is a vertical partition of the table.

Consider the following queries.

Eno, Ename EMP

What does the relational algebraic expression derive?

It is going to project the columns Eno, Ename from the EMP table
Name ((Dno = 6 ) EMP)

First, the inner query will get executed
Assignment Operator (): We can show the sequence of operations by giving a name to each intermediate relation with the assignment operator.

Though the assignment operator is a basic operator we will have a discussion here which helps in understanding the further topics easily.

Example: DEPT_5  (Dnum = 5) Dept

Here the output of the query (Dnum = 5) Dept will be assigned to DEPT_5.

In our further discussions, we cover more examples of this operator.

In our next discussion, we will continue Introduction to Relational Algebra and its operators in DBMS on remaining operators.





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