You are permitted to specify DUAL as a dummy table name in situations
where no tables are referenced:

mysql> SELECT 1 + 1 FROM DUAL;
        -> 2

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

URL: http://dev.mysql.com/doc/refman/5.5/en/select.html

