SQL Server UNION Operator

In SQL Server, the UNION operator is used to combine the result-set of two or more SELECT statements.

Syntax:

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION      
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]; 

Parameter explanation

expression1, expression2, ... expression_n: expressions specify the columns or calculations that you want to compare between the two SELECT statements.

tables: It specifies the tables that you want to retrieve records from. There must be at least one table listed in the FROM clause.

WHERE conditions: It is optional condition. It specifies the conditions that must be met for the records to be selected.

Image representation:

Note: The covered blue area specifies the union data.

UNION operator with single expression

Example:

SELECT name
FROM [javatpoint].[dbo].[Employees]
UNION
SELECT name
FROM [javatpoint].[dbo].[Employee2]; 

Output:

UNION operator with multiple expressions

Example:

Let's use multiple expressions of each table. For example: id, name, salary.

SELECT id, name, salary
FROM [javatpoint].[dbo].[Employees]
WHERE salary >= 15000
UNION
SELECT id, name, salary
FROM [javatpoint].[dbo].[Employee2]

Output:

UNION ALL Operator

The UNION operator selects only distinct values by default. So, the UNION ALL operator is used to allow duplicate values also.

SELECT column_name(s) FROM table1
UNION ALL
SELECT column_name(s) FROM table2; 

Example:

SELECT id, name, salary
FROM [javatpoint].[dbo].[Employees]
WHERE salary >= 15000
UNION
SELECT id, name, salary
FROM [javatpoint].[dbo].[Employee2]

Output:

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