1.问题描述：
数据表结构同Mysql笔记（一），需求改为查找给定分类编码查找分类及其所有子类，包含子类的子类。
2.问题分析：
还是利用分类编码具有层次嵌套的特点，这一次需要按照org_code升序排列，目的是保障一个分类的子类一定出现在其后面。然后可以利用笔记（一）中相同的逻辑遍历每一个分类，关键点是需要不断扩充被查找的父类集合，然后利用find_in_set函数进行过滤。
（1）按org_code升序排列：
mysql> select org_code,parent_code from class order by org_code asc; +------------------+--------------+ | org_code | parent_code | +------------------+--------------+ | 0001 | NULL | | 00010001 | 0001 | | 000100010001 | 00010001 | | 000100010002 | 00010001 | | 0001000100020001 | 000100010002 | | 00010002 | 0001 | | 00010003 | 0001 | +------------------+--------------+ 7 rows in set (0.00 sec)
（2）遍历排序后的数据，将待查寻父类的子类逐步添加到待查寻父类列表@p中：
mysql> select (case when org_code=@r or find_in_set(parent_code,@p) then org_code else null end) as child,(case when find_in_set(parent_code,@p) then @p:=concat(@p,',',org_code) else null end) as parents from (select org_code,parent_code from class order by org_code asc) as c,(select @r:='00010001',@p:='00010001') as t; +------------------+-----------------------------------------------------+ | child | parents | +------------------+-----------------------------------------------------+ | NULL | NULL | | 00010001 | NULL | | 000100010001 | 00010001,000100010001 | | 000100010002 | 00010001,000100010001,000100010002 | | 0001000100020001 | 00010001,000100010001,000100010002,0001000100020001 | | NULL | NULL | | NULL | NULL | +------------------+-----------------------------------------------------+ 7 rows in set (0.00 sec)
（3）过滤查询结果，得到分类编号为00010001的分类及其子类的子类：
mysql> select child from (select (case when org_code=@r or find_in_set(parent_code,@p) then org_code else null end) as child,(case when find_in_set(parent_code,@p) then @p:=concat(@p,',',org_code) else null end) as parents from (select org_code,parent_code from class order by org_code asc) as c,(select @r:='00010001',@p:='00010001') as t) as m where child is not null; +------------------+ | child | +------------------+ | 00010001 | | 000100010001 | | 000100010002 | | 0001000100020001 | +------------------+ 4 rows in set (0.00 sec)