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 −

Table R2 is as follows −

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

∏regno(R1) ∪ ∏regno(R2)

Output

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

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

Output

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.