UNION

SELECT ...
UNION [ALL | DISTINCT] SELECT ...
[UNION [ALL | DISTINCT] SELECT ...]

A UNION statement combines the results from multiple SELECT statements.

To use an ORDER BY or LIMIT clause for the entire UNION result, parenthesize the individual SELECT statements and place the ORDER BY or LIMIT clause at the end:

(SELECT a FROM t1 WHERE a=100 AND b=5)
UNION
(SELECT a FROM t2 WHERE a=101 AND b=3)
ORDER BY a LIMIT 15;