SQL UNION ALL operator

Posted by : Glen Sajori Sabtu, 27 September 2014

Description

The SQL 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.

Syntax

The syntax for the SQL UNION ALL operator is:

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

Parameters or Arguments

expression1, expression2, expression_n are the columns or calculations that you wish to retrieve.

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

conditions are conditions that must be met for the records to be selected.

Note

There must be same number of expressions in both SELECT statements.
See also the UNION operator.

Example - Return single field

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

SELECT supplier_id
FROM suppliers
UNION ALL
SELECT supplier_id
FROM orders;

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

Example - Using SQL ORDER BY

The UNION ALL operator can use the ORDER BY clause to order the results of the query.
For example:

SELECT supplier_id, supplier_name
FROM suppliers
WHERE supplier_id > 2000
UNION ALL
SELECT company_id, company_name
FROM companies
WHERE company_id > 1000
ORDER BY 2;

In this SQL UNION ALL 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 supplier_name / company_name in ascending order, as denoted by the "ORDER BY 2".

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

Source : techonthenet.com