SQL Server: UNION Operator

In this post explains how to use the UNION operator in SQL Server (Transact-SQL) with syntax and examples.

Description

The SQL Server UNION operator is used to combine the result sets of 2 or more SELECT statements. It removes duplicate rows between the various SELECT statements.

Each SELECT statement within the UNION operator must have the same number of columns in the result sets with similar data types.

Syntax

The syntax for the UNION operator in SQL Server (Transact-SQL) is:

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

Parameters or Arguments

expression1, expression2, ... expression_n

The columns or calculations that you wish to retrieve.

tables

The tables that you wish to retrieve records from. There must be at least one table listed in the FROM clause.

WHERE conditions

Optional. The conditions that must be met for the records to be selected.

Note

• There must be same number of expressions in both SELECT statements.
• The corresponding columns in each of the SELECT statements must have similar data types.
• The UNION operator removes duplicate rows.
• See also the UNION ALL operator.

Example - Return single field

Let's look at an example of the SQL Server UNION operator that returns one field from multiple SELECT statements (and both fields have the same data type).

For example:

SELECT product_id
FROM products
UNION
SELECT product_id
FROM inventory;

In this SQL Server UNION example, if a product_id appeared in both the products and inventory tables, it would appear once in your result set. The UNION operator removes duplicates in SQL Server. If you do not wish to remove duplicates, try using the UNION ALL operator.

Example - Using ORDER BY

The UNION operator can use the ORDER BY clause to order the results of the query in SQL Server (Transact-SQL).

For example:

SELECT contact_id, contact_name
FROM contacts
WHERE site_name = 'AODBA.com'
UNION
SELECT company_id, company_name
FROM companies
WHERE site_name = 'mySite.com'
ORDER BY 2;

In this UNION example, since the column names are different between the two SELECT statements, it is more advantageous to reference the columns in the ORDER BY clause by their position in the result set. In this example, we've sorted the results by contact_name / company_name in ascending order, as denoted by the ORDER BY 2.

The contact_name / company_name fields are in position #2 in the result set.