

Parentheses for specifying precedence, from MariaDB 10.4.0

CREATE OR REPLACE TABLE t1 (a INT);
CREATE OR REPLACE TABLE t2 (b INT);
CREATE OR REPLACE TABLE t3 (c INT);

INSERT INTO t1 VALUES (1),(2),(3),(4);
INSERT INTO t2 VALUES (5),(6);
INSERT INTO t3 VALUES (1),(6);

((SELECT a FROM t1) UNION (SELECT b FROM t2)) INTERSECT (SELECT c FROM t3);
+------+
| a    |
+------+
|    1 |
|    6 |
+------+

(SELECT a FROM t1) UNION ((SELECT b FROM t2) INTERSECT (SELECT c FROM t3));
+------+
| a    |
+------+
|    1 |
|    2 |
|    3 |
|    4 |
|    6 |
+------+

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