MariaDB: UNION ALL Operator

This MariaDB tutorial explains how to use the MariaDB UNION ALL operator with syntax and examples.

Description

The MariaDB UNION ALL operator is used to combine the result sets of 2 or more SELECT statements. It returns all rows from the query and it does not remove duplicate rows between the various SELECT statements.

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

Syntax

The syntax for the UNION ALL operator in MariaDB 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

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 column names from the first SELECT statement are used as the column names for the result set.

Example - Return single field

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

SELECT site_id
FROM sites
UNION ALL
SELECT site_id
FROM pages;

This MariaDB UNION ALL operator would return a site_id multiple times in your result set if the site_id appeared in both the sites and pages table. The MariaDB UNION ALL operator does not remove duplicates. If you wish to remove duplicates, try using the MariaDB UNION operator.

Example - Using ORDER BY

The MariaDB UNION ALL operator can use the ORDER BY clause to order the results of the operator.

For example:

SELECT site_id, site_name
FROM sites
WHERE site_name = 'AODBA.com'
UNION ALL
SELECT page_id, page_title
FROM pages
WHERE page_id > 10
ORDER BY 2;

In this MariaDB UNION ALL operator, 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 site_name / page_title in ascending order, as denoted by the ORDER BY 2.

The site_name / page_title fields are in position #2 in the result set.