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# Explain project operation in relational algebra (DBMS)?

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:

Regno | Branch | Section |
---|---|---|

1 | CSE | A |

2 | ECE | B |

3 | CIVIL | B |

4 | IT | A |

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 −

Branch | Section |
---|---|

CSE | A |

ECE | B |

CIVIL | B |

IT | A |

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

∏regno,section(σbranch=ECE(student))

### Output

Regno | Section |
---|---|

2 | B |

**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)

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