题解 | 统计所有课程参加培训人次

SELECT 
    (SELECT COUNT(DISTINCT staff_id) FROM cultivate_tb WHERE FIND_IN_SET('course1', course)) +
    (SELECT COUNT(DISTINCT staff_id) FROM cultivate_tb WHERE FIND_IN_SET('course2', course)) +
    (SELECT COUNT(DISTINCT staff_id) FROM cultivate_tb WHERE FIND_IN_SET('course3', course)) 
    AS staff_nums;