Oracle 中的树查询和 connect by

Oracle 中的树查询和 connect by
使用 connect by 和 start with 来建立类似于树的报表并不难，只要遵循以下基本原则即可：
使用 connect by 时各子句的顺序应为：
select
from
where
start with
connect by
order by
prior 使报表的顺序为从根到叶（如果 prior 列是父辈）或从叶到根（如果 prior 列是后代）。
where 子句可以从树中排除个体，但不排除它们的子孙（或者祖先，如果 prior 列是后代）。
connect by 中的条件（尤其是不等于）消除个体和它所有的子孙（或祖先，依赖于怎样跟踪树）。
connect by 不能与 where 子句中的表连接在一起使用。
 
下面是几个例子
1. 从根到叶遍历
SELECT n_parendid, n_name, (LEVEL - 1), n_id
from navigation
WHERE n_parendid IS NOT NULL
START WITH n_id = 0
CONNECT BY n_parendid = PRIOR n_id;
2. 从叶到根遍历
SELECT n_parendid, n_name, (LEVEL - 1), n_id
from navigation
WHERE n_parendid IS NOT NULL
START WITH n_id = 300
CONNECT BY n_id = PRIOR n_parendid;
3. 排除个体，但不排除它们的子孙
SELECT n_parendid, n_name, (LEVEL - 1), n_id
from navigation
WHERE n_parendid IS NOT NULL AND n_id != 2
START WITH n_id = 0
CONNECT BY n_parendid = PRIOR n_id;
4. 消除个体和它所有的子孙
SELECT n_parendid, n_name, (LEVEL - 1), n_id
from navigation
WHERE n_parendid IS NOT NULL
START WITH n_id = 0
CONNECT BY n_parendid = PRIOR n_id AND n_id != 2;
5. 改变显示顺序
SELECT n_parendid, n_name, (LEVEL - 1), n_id
from navigation
WHERE n_parendid IS NOT NULL
START WITH n_id = 0
CONNECT BY n_parendid = PRIOR n_id
ORDER BY n_viewnum DESC; 
本文转自csdn:http://blog.csdn.net/wzy0623/archive/2007/06/18/1656345.aspx