MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE `test_set` (
  `int` int(11) DEFAULT NULL,
  `set_2` set('a','b','c') DEFAULT NULL
)
INSERT INTO `test_set` VALUES (0,0), (1,1), (2,2), (3,3), (4,4), (5,5), (6,6), (7,7)
SELECT DISTINCT `int`, SUBSTRING_INDEX(SUBSTRING_INDEX(`set_2`, ',', `mask`), ',', -1) `set`
FROM `test_set`, 
     (SELECT 1 `mask` UNION 
      SELECT 2 UNION 
      SELECT 3) `mask`
WHERE `set_2` & 1;


int
set


1
a

3
a

3
b

5
a

5
c

7
a

7
b

7
c