SQL UNION Operator

The SQL UNION Operator

The UNION operator is used to combine the result-set of two or more SELECT statements.

• Every SELECT statement within UNION must have the same number of columns
• The columns must also have similar data types
• The columns in every SELECT statement must also be in the same order

UNION Syntax

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

UNION ALL Syntax

The UNION operator selects only distinct values by default. To allow duplicate values, use UNION ALL:

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

Note: The column names in the result-set are usually equal to the column names in the first SELECT statement.

SQL UNION Example

The following SQL statement returns the cities (only distinct values) from both the “Customers” and the “Suppliers” table:

SELECT City FROM Customers
UNION
SELECT City FROM Suppliers
ORDER BY City;  

Note: If some customers or suppliers have the same city, each city will only be listed once, because UNION selects only distinct values. Use UNION ALL to also select duplicate values!

SQL UNION ALL Example

The following SQL statement returns the cities (duplicate values also) from both the “Customers” and the “Suppliers” table:

SELECT City FROM Customers
UNION ALL
SELECT City FROM Suppliers
ORDER BY City; 

SQL UNION With WHERE

The following SQL statement returns the German cities (only distinct values) from both the “Customers” and the “Suppliers” table:

SELECT City, Country FROM Customers
WHERE Country='Germany'
UNION
SELECT City, Country FROM Suppliers
WHERE Country='Germany'
ORDER BY City;

SQL UNION ALL With WHERE

The following SQL statement returns the German cities (duplicate values also) from both the “Customers” and the “Suppliers” table:

SELECT City, Country FROM Customers
WHERE Country='Germany'
UNION ALL
SELECT City, Country FROM Suppliers
WHERE Country='Germany'
ORDER BY City;

Another UNION Example

The following SQL statement lists all customers and suppliers:

SELECT 'Customer' AS Type, ContactName, City, Country
FROM Customers
UNION
SELECT 'Supplier', ContactName, City, Country
FROM Suppliers;

Notice the “AS Type” above - it is an alias. SQL Aliases are used to give a table or a column a temporary name. An alias only exists for the duration of the query. So, here we have created a temporary column named “Type”, that list whether the contact person is a “Customer” or a “Supplier”.