Syntax
------

SELECT ... WHERE EXISTS <Table subquery>

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

Subqueries using the EXISTS keyword will return true if the subquery returns
any rows. Conversely, subqueries using NOT EXISTS will return true only if the
subquery returns no rows from the table.

EXISTS subqueries ignore the columns specified by the SELECT of the subquery,
since they're not relevant. For example,

SELECT col1 FROM t1 WHERE EXISTS (SELECT * FROM t2);

and

SELECT col1 FROM t1 WHERE EXISTS (SELECT col2 FROM t2);

produce identical results.

Examples
--------

CREATE TABLE sq1 (num TINYINT);

CREATE TABLE sq2 (num2 TINYINT);

INSERT INTO sq1 VALUES(100);

INSERT INTO sq2 VALUES(40),(50),(60);

SELECT * FROM sq1 WHERE EXISTS (SELECT * FROM sq2 WHERE num2>50);
+------+
| num  |
+------+
|  100 |
+------+

SELECT * FROM sq1 WHERE NOT EXISTS (SELECT * FROM sq2 GROUP BY num2 HAVING
MIN(num2)=40);
Empty set (0.00 sec)

URL: https://mariadb.com/kb/en/subqueries-and-exists/