mysql 查询所有子节点

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 := '' college_code ) t2
    ) t3
WHERE
    t3.ischild != 0

表结构：  t_college:      id,   college_code (机构编码),   parent_org_code （父机构编码）