Mysql笔记（二）：递归分类层次查询

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)