SQL Keyword - UNION And UNION ALL

In this blog, we learn about SQL Keyword - UNION And UNION ALL.

UNION Keyword

The result set from two or more SELECT operations is combined by the UNION command but only distinct values.

There are two basic rules are followed when you want to use Union Keyword or Union All.

1. All queries must have the same amount of columns and column ordering.
2. Each query requires that the data types of the columns on the involving table be the same or compatible.

Syntax

SELECT <COLUMN1>,<COLUMN2> FROM <TABLE1>
UNION
SELECT <COLUMN1>,<COLUMN2> FROM <TABLE2>

Example

SELECT Name FROM Employee
UNION
SELECT Name FROM Worker

In the above example, it returns only unique names from both tables.

UNION ALL Keyword

The result set from two or more SELECT operations is combined by the UNION ALL command, and it also allows duplicate values.

Syntax

SELECT <COLUMN1>,<COLUMN2> FROM <TABLE1>
UNION ALL
SELECT <COLUMN1>,<COLUMN2> FROM <TABLE2>

Example

SELECT Name FROM Employee
UNION ALL
SELECT Name FROM Worker

In the above example, it returns all names from both tables, and if names are duplicates, it also returns.

Summary

The UNION and UNION ALL command combine the result set of two or more SELECT statements.