Description
-----------

You are allowed to specify DUAL as a dummy table name in situations where no
tables are referenced, such as the following SELECT statement:

SELECT 1 + 1 FROM DUAL;
+-------+
| 1 + 1 |
+-------+
|     2 |
+-------+

DUAL is purely for the convenience of people who require that all SELECT
statements should have FROM and possibly other clauses. MariaDB ignores the
clauses. MariaDB does not require FROM DUAL if no tables are referenced.

FROM DUAL could be used when you only SELECT computed values, but require a
WHERE clause, perhaps to test that a script correctly handles empty resultsets:

SELECT 1 FROM DUAL WHERE FALSE;
Empty set (0.00 sec)

URL: https://mariadb.com/kb/en/dual/