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# Explain union 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

Union, intersection, difference, cartesian, join, division comes under binary operation (operate on two tables).

## Union operation

Union operation combines values in R1, R2 by removing duplicate ones.

### Syntax

∏regno(R1) ∪ ∏regno(R2)

It displays all the regno of R1 and R2.

### Example

Consider two tables R1 and R2 −

Table R1 is as follows −

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

1 | CSE | A |

2 | ECE | B |

3 | MECH | B |

4 | CIVIL | A |

5 | CSE | B |

**Table R2** is as follows −

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

1 | CIVIL | A |

2 | CSE | A |

3 | ECE | B |

To display all the regno of R1 and R2, use the following command −

∏regno(R1) ∪ ∏regno(R2)

### Output

Regno |
---|

1 |

2 |

3 |

4 |

5 |

To retrieve branch and section of student from table R1 and R2, use the below mentioned command −

∏ branch, section (R1) ∪ ∏ branch, section (R2)

### Output

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

CSE | A |

ECE | B |

MECH | B |

CIVIL | A |

CSE | B |

No repletion allowed for entry.

**Union All** − It is used for duplicate entries. The common entries will be shown if the common intersection is used.

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