Oracle中的select语句可以用start with...connect by prior子句实现递归查询，connect by 是结构化查询中用到的，其基本语法是：
select ...
from <TableName>
where <Conditional-1>
start with <Conditional-2>
connect by <Conditional-3>;
说明：
<Conditional-1>：过滤条件，用于对返回的所有记录进行过滤。
<Conditional-2>：查询结果重起始根结点的限定条件。
<Conditional-3>：连接条件
还是看个例子吧，应该能更好的说明问题。数据库表结构如下：
create table t2(
root_id number,
id number,
name varchar(5),
description varchar(10)
);
插入测试数据：
insert into t2(root_id,id,name,description) values(0,1,'a','aaa');
insert into t2(root_id,id,name,description) values(1,2,'a1','aaa1');
insert into t2(root_id,id,name,description) values(1,3,'a2','aaa2');
insert into t2(root_id,id,name,description) values(0,4,'b','bbb');
insert into t2(root_id,id,name,description) values(4,5,'b1','bbb1');
insert into t2(root_id,id,name,description) values(4,6,'b2','bbb2');
---------------------------------------------------------------------------------------------------
--------------------------------------准备好了，开始吧-----------------------------------------
---------------------------------------------------------------------------------------------------
1. 获取完整树：
select * from t2 start with root_id = 0 connect by prior id = root_id;
结构图：
|---------a------a1
| |
| |
| |------a2
|
|---------b------b1
| |
| |
| |------b2
2. 获取特定子树：
2.1 select * from t2 start with id = 1 connect by prior id = root_id;
结构图：
|---------a------a1
| |
| |
| |------a2
2.2 select * from t2 start with id = 4 connect by prior id = root_id;
结构图：
|---------b------b1
| |
| |
| |------b2
说明：如果connect by prior中的prior被省略，则查询将不进行深层递归。
例如：
select * from t2 start with root_id = 0 connect by id = root_id;
|---------a
|
|
|---------b
select * from t2 start with id = 1 connect by id = root_id;
|---------a
以上内容从其他网友那里copy加工而成，特别感谢实例的原创者！！！