mysql 查詢所有子節點

SELECT
    t3.college_code 
FROM
    (
    SELECT
    t1.college_code,
IF
    ( find_in_set( t1.parent_org_code, @pids ) > 0, @pids := concat( @pids, ',', t1.college_code ), 0 ) AS ischild 
FROM
    ( SELECT college_code, parent_org_code FROM t_college t WHERE t.STATUS = 1 ORDER BY parent_org_code, college_code ) t1,
    ( SELECT @pids := '22300' college_code ) t2 
    ) t3 
WHERE
    t3.ischild != 0





表結構：  t_college:      id,   college_code (機構編碼),   parent_org_code （父機構編碼）