TSQL Tutorial : UNION ALL Operator

The TSQL UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It returns all rows from the query (even if the row exists in more than one of the SELECT statements).

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

The syntax for the TSQL UNION ALL operator is:

SELECT expression1, expression2, ... expression_n

FROM tables

WHERE conditions

UNION ALL

SELECT expression1, expression2, ... expression_n

FROM tables

WHERE conditions;

There must be same number of expressions in both SELECT statements.

Return single field

The following is an example of the TSQL UNION ALL operator that returns one field from multiple SELECT statements (and both fields have the same data type):

SELECT customer_id

FROM customer

UNION ALL

SELECT customer_id

FROM orders;

This TSQL UNION ALL example would return a customer_id multiple times in your result set if the customer_id appeared in both the suppliers and orders table. The TSQL UNION ALL operator does not remove duplicates. If you wish to remove duplicates, try using the UNION operator.