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 −
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
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.
Published on 06-Jul-2021 14:23:11
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