UNION (Transact-SQL) - SQL Server (2023)

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW)

Concatenates the results of two queries into a single result set. You control whether the result set includes duplicate rows:

• UNION ALL - Includes duplicates.
• UNION - Excludes duplicates.

A UNION operation is different from a JOIN:

• A UNION concatenates result sets from two queries. But a UNION does not create individual rows from columns gathered from two tables.
• A JOIN compares columns from two tables, to create result rows composed of columns from two tables.

The following are basic rules for combining the result sets of two queries by using UNION:

• The number and the order of the columns must be the same in all queries.

• The data types must be compatible.

Transact-SQL syntax conventions

Syntax

{ <query_specification> | ( <query_expression> ) } { UNION [ ALL ] { <query_specification> | ( <query_expression> ) } [ ...n ] }

Arguments

<query_specification> | ( <query_expression> )Is a query specification or query expression that returns data to be combined with the data from another query specification or query expression. The definitions of the columns that are part of a UNION operation don't have to be the same, but they must be compatible through implicit conversion. When data types differ, the resulting data type is determined based on the rules for data type precedence. When the types are the same but differ in precision, scale, or length, the result is based on the same rules for combining expressions. For more information, see Precision, Scale, and Length (Transact-SQL).

Columns of the xml data type must be equal. All columns must be either typed to an XML schema or untyped. If typed, they must be typed to the same XML schema collection.

UNION
Specifies that multiple result sets are to be combined and returned as a single result set.

ALL
Incorporates all rows into the results, including duplicates. If not specified, duplicate rows are removed.

Examples

A. Using a simple UNION

In the following example, the result set includes the contents of the ProductModelID and Name columns of both the ProductModel and Gloves tables.

-- Uses AdventureWorks IF OBJECT_ID ('dbo.Gloves', 'U') IS NOT NULL DROP TABLE dbo.Gloves; GO -- Create Gloves table. SELECT ProductModelID, Name INTO dbo.Gloves FROM Production.ProductModel WHERE ProductModelID IN (3, 4); GO -- Here is the simple union. -- Uses AdventureWorks SELECT ProductModelID, Name FROM Production.ProductModel WHERE ProductModelID NOT IN (3, 4) UNION SELECT ProductModelID, Name FROM dbo.Gloves ORDER BY Name; GO 

B. Using SELECT INTO with UNION

In the following example, the INTO clause in the second SELECT statement specifies that the table named ProductResults holds the final result set of the union of the selected columns of the ProductModel and Gloves tables. The Gloves table is created in the first SELECT statement.

-- Uses AdventureWorks IF OBJECT_ID ('dbo.ProductResults', 'U') IS NOT NULL DROP TABLE dbo.ProductResults; GO IF OBJECT_ID ('dbo.Gloves', 'U') IS NOT NULL DROP TABLE dbo.Gloves; GO -- Create Gloves table. SELECT ProductModelID, Name INTO dbo.Gloves FROM Production.ProductModel WHERE ProductModelID IN (3, 4); GO -- Uses AdventureWorks SELECT ProductModelID, Name INTO dbo.ProductResults FROM Production.ProductModel WHERE ProductModelID NOT IN (3, 4) UNION SELECT ProductModelID, Name FROM dbo.Gloves; GO SELECT ProductModelID, Name FROM dbo.ProductResults; 

C. Using UNION of two SELECT statements with ORDER BY

The order of certain parameters used with the UNION clause is important. The following example shows the incorrect and correct use of UNION in two SELECT statements in which a column is to be renamed in the output.

-- Uses AdventureWorks IF OBJECT_ID ('dbo.Gloves', 'U') IS NOT NULL DROP TABLE dbo.Gloves; GO -- Create Gloves table. SELECT ProductModelID, Name INTO dbo.Gloves FROM Production.ProductModel WHERE ProductModelID IN (3, 4); GO /* INCORRECT */ -- Uses AdventureWorks SELECT ProductModelID, Name FROM Production.ProductModel WHERE ProductModelID NOT IN (3, 4) ORDER BY Name UNION SELECT ProductModelID, Name FROM dbo.Gloves; GO /* CORRECT */ -- Uses AdventureWorks SELECT ProductModelID, Name FROM Production.ProductModel WHERE ProductModelID NOT IN (3, 4) UNION SELECT ProductModelID, Name FROM dbo.Gloves ORDER BY Name; GO 

D. Using UNION of three SELECT statements to show the effects of ALL and parentheses

The following examples use UNION to combine the results of three tables that all have the same 5 rows of data. The first example uses UNION ALL to show the duplicated records, and returns all 15 rows. The second example uses UNION without ALL to eliminate the duplicate rows from the combined results of the three SELECT statements, and returns 5 rows.

The third example uses ALL with the first UNION and parentheses enclose the second UNION that isn't using ALL. The second UNION is processed first because it's in parentheses, and returns 5 rows because the ALL option isn't used and the duplicates are removed. These 5 rows are combined with the results of the first SELECT by using the UNION ALL keywords. This example doesn't remove the duplicates between the two sets of five rows. The final result has 10 rows.

-- Uses AdventureWorks IF OBJECT_ID ('dbo.EmployeeOne', 'U') IS NOT NULL DROP TABLE dbo.EmployeeOne; GO IF OBJECT_ID ('dbo.EmployeeTwo', 'U') IS NOT NULL DROP TABLE dbo.EmployeeTwo; GO IF OBJECT_ID ('dbo.EmployeeThree', 'U') IS NOT NULL DROP TABLE dbo.EmployeeThree; GO SELECT pp.LastName, pp.FirstName, e.JobTitle INTO dbo.EmployeeOne FROM Person.Person AS pp JOIN HumanResources.Employee AS e ON e.BusinessEntityID = pp.BusinessEntityID WHERE LastName = 'Johnson'; GO SELECT pp.LastName, pp.FirstName, e.JobTitle INTO dbo.EmployeeTwo FROM Person.Person AS pp JOIN HumanResources.Employee AS e ON e.BusinessEntityID = pp.BusinessEntityID WHERE LastName = 'Johnson'; GO SELECT pp.LastName, pp.FirstName, e.JobTitle INTO dbo.EmployeeThree FROM Person.Person AS pp JOIN HumanResources.Employee AS e ON e.BusinessEntityID = pp.BusinessEntityID WHERE LastName = 'Johnson'; GO -- Union ALL SELECT LastName, FirstName, JobTitle FROM dbo.EmployeeOne UNION ALL SELECT LastName, FirstName ,JobTitle FROM dbo.EmployeeTwo UNION ALL SELECT LastName, FirstName,JobTitle FROM dbo.EmployeeThree; GO SELECT LastName, FirstName,JobTitle FROM dbo.EmployeeOne UNION SELECT LastName, FirstName, JobTitle FROM dbo.EmployeeTwo UNION SELECT LastName, FirstName, JobTitle FROM dbo.EmployeeThree; GO SELECT LastName, FirstName,JobTitle FROM dbo.EmployeeOne UNION ALL ( SELECT LastName, FirstName, JobTitle FROM dbo.EmployeeTwo UNION SELECT LastName, FirstName, JobTitle FROM dbo.EmployeeThree ); GO 

Examples: Azure Synapse Analytics and Analytics Platform System (PDW)

E. Using a simple UNION

In the following example, the result set includes the contents of the CustomerKey columns of both the FactInternetSales and DimCustomer tables. Since the ALL keyword isn't used, duplicates are excluded from the results.

-- Uses AdventureWorks SELECT CustomerKey FROM FactInternetSales UNION SELECT CustomerKey FROM DimCustomer ORDER BY CustomerKey; 

F. Using UNION of two SELECT statements with ORDER BY

When any SELECT statement in a UNION statement includes an ORDER BY clause, that clause should be placed after all SELECT statements. The following example shows the incorrect and correct use of UNION in two SELECT statements in which a column is ordered with ORDER BY.

-- Uses AdventureWorks -- INCORRECT SELECT CustomerKey FROM FactInternetSales ORDER BY CustomerKey UNION SELECT CustomerKey FROM DimCustomer ORDER BY CustomerKey; -- CORRECT USE AdventureWorksPDW2012; SELECT CustomerKey FROM FactInternetSales UNION SELECT CustomerKey FROM DimCustomer ORDER BY CustomerKey; 

G. Using UNION of two SELECT statements with WHERE and ORDER BY

The following example shows the incorrect and correct use of UNION in two SELECT statements where WHERE and ORDER BY are needed.

-- Uses AdventureWorks -- INCORRECT SELECT CustomerKey FROM FactInternetSales WHERE CustomerKey >= 11000 ORDER BY CustomerKey UNION SELECT CustomerKey FROM DimCustomer ORDER BY CustomerKey; -- CORRECT USE AdventureWorksPDW2012; SELECT CustomerKey FROM FactInternetSales WHERE CustomerKey >= 11000 UNION SELECT CustomerKey FROM DimCustomer ORDER BY CustomerKey; 

H. Using UNION of three SELECT statements to show effects of ALL and parentheses

The following examples use UNION to combine the results of the same table to demonstrate the effects of ALL and parentheses when using UNION.

The first example uses UNION ALL to show duplicated records and returns each row in the source table three times. The second example uses UNION without ALL to eliminate the duplicate rows from the combined results of the three SELECT statements and returns only the unduplicated rows from the source table.

The third example uses ALL with the first UNION and parentheses enclosing the second UNION that isn't using ALL. The second UNION is processed first because it is in parentheses. It returns only the unduplicated rows from the table because the ALL option isn't used and duplicates are removed. These rows are combined with the results of the first SELECT by using the UNION ALL keywords. This example doesn't remove the duplicates between the two sets.

-- Uses AdventureWorks SELECT CustomerKey, FirstName, LastName FROM DimCustomer UNION ALL SELECT CustomerKey, FirstName, LastName FROM DimCustomer UNION ALL SELECT CustomerKey, FirstName, LastName FROM DimCustomer; SELECT CustomerKey, FirstName, LastName FROM DimCustomer UNION SELECT CustomerKey, FirstName, LastName FROM DimCustomer UNION SELECT CustomerKey, FirstName, LastName FROM DimCustomer; SELECT CustomerKey, FirstName, LastName FROM DimCustomer UNION ALL ( SELECT CustomerKey, FirstName, LastName FROM DimCustomer UNION SELECT CustomerKey, FirstName, LastName FROM DimCustomer ); 

See Also

SELECT (Transact-SQL)
SELECT Examples (Transact-SQL)