Explain the SELECT Operation in Relational Algebra

Query is a question or requesting information. Query language is a language which is used to retrieve information from a database.

Query language is divided into two types −

• Procedural language

• Non-procedural language

Procedural language

Information is retrieved from the database by specifying the sequence of operations to be performed.

For Example − Relational algebra.

Structure Query language (SQL) is based on relational algebra.

Relational algebra consists of a set of operations that take one or two relations as an input and produces a new relation as output.

Types of Relational Algebra operations

The different types of relational algebra operations are as follows −

• Select operation

• Project operation

• Rename operation

• Union operation

• Intersection operation

• Difference operation

• Cartesian product operation

• Join operation

• Division operation

Select, project, rename comes under unary operation (operate on one table).

Select operation

It displays the records that satisfy a condition. It is denoted by sigma (σ) and is a horizontal subset of the original relation.

Syntax

Its syntax is as follows −

σcondition(table name)

Example

Consider the student table given below −

Now, to display all the records of student table, we will use the following command −

σ(student)

In addition to this, when we have to display all the records of CSE branch in student table, we will use the following command −

σbranch=cse(student)

Hence, the result will be as follows −

To display all the records in student tables whose regno>2, we will use the below mentioned command −

σRegNo>2(student)

The output will be as follows −

To display the record of ECE branch section B students, use the given command −

σbranch=ECE ^ section=B(student)

To display the records of section B CSE and IT branch, use the following command −

σSection=B ^ Branch=cse ∨ branch=IT(student)

Consider the EMPLOYEE TABLE as another example to know about selection operations.

Retrieve information about those employees whose salary is greater than 20,000

• If one condition is specified then, we can use the following command −

σ salary > 20,000 (emp)

• If more than one condition specified in the query then ( AND: ^, OR:∨ , Not:#, equal: =, >, <, >=, <=)

Relational operator will be used to combine the multiple conditions into a single statement.

Example − In order to retrieve the information of those employee whose salary > 20,000 and working in HOD and Dept no is 20, we can use the following command −

σ salary > 20,000 ^LOC=HOD ^Deptno=20(emp)