Explain union 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

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 −

RegnoBranchSection
1CSEA
2ECEB
3MECHB
4CIVILA
5CSEB

Table R2 is as follows −

RegnoBranchSection
1CIVILA
2CSEA
3ECEB

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

BranchSection
CSEA
ECEB
MECHB
CIVILA
CSEB

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.

raja
Published on 06-Jul-2021 14:23:11
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