Explain project operation in relational algebra (DBMS)?

DBMSDatabaseBig Data Analytics

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).

Projection operation

It displays the specific column of a table. It is denoted by pie (∏). It is a vertical subset of the original relation. It eliminates duplicate tuples.

Syntax

The syntax is as follows −

regno(student)

Example

Consider the student table:

RegnoBranchSection
1CSEA
2ECEB
3CIVILB
4ITA

To display regno column of student table, we can use the following command −

regno(student)

Output

RegNo
1
2
3
4

To display branch, section column of student table, use the following command −

∏branch,section(student)

The result is as follows −

BranchSection
CSEA
ECEB
CIVILB
ITA

To display regno, section of ECE students, use the following command −

∏regno,section(σbranch=ECE(student))

Output

RegnoSection
2B

Note: Conditions can be written in select operation but not in projection operation.

Consider the employee table to know more about projection.

  • If no condition is specified in the query then, Π empid, ename, salary, address, dno (emp).

  • If condition is specified then, the composition of the select and projection is as follows −

∏ empid, ename, salary, address, dno (σ salary >20,00 ^ LOC = HOD ^ dno=20) (emp)
raja
Published on 06-Jul-2021 14:14:13
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