CREATE TABLE t1 (
id int NOT NULL AUTO_INCREMENT,
a int NOT NULL,
b int NOT NULL,
PRIMARY KEY (id),
UNIQUE KEY ba (b, a)
) ENGINE=InnoDB;
INSERT INTO t1 (a, b) VALUES(1,101),(1,102),(1,103),(1,104),(1,105),(1,106),(1,107),(1,108),(1,109),(1,110);
SELECT COUNT(DISTINCT b) FROM t1 WHERE a = 1;
COUNT(DISTINCT b)
10
DROP TABLE t1;