Oracle中的Connect By 用法

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>
;

<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

数据组织结构如下图：

|---a------a1 
|     |------a2
|
|---b------b1
      |------b2

数据库表结构如下：

create table t2(
root_id number,
id number,
name varchar(5),
description varchar(10)
);

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');

SQL> select * from t2;

   ROOT_ID         ID NAME DESCRIPTIO
---------- ---------- ----- ----------
         0          1 a     aaa
         1          2 a1    aaa1
         1          3 a2    aaa2
         0          4 b     bbb
         4          5 b1    bbb1
         4          6 b2    bbb2
         
获取完整树：
select * from t2 start with root_id = 0 connect by prior id = root_id;

|---a------a1 
|     |------a2
|
|---b------b1
      |------b2

   ROOT_ID         ID NAME DESCRIPTIO
---------- ---------- ----- ----------
         0          1 a     aaa
         1          2 a1    aaa1
         1          3 a2    aaa2
         0          4 b     bbb
         4          5 b1    bbb1
         4          6 b2    bbb2

获取特定子树：
select * from t2 start with id = 1 connect by prior id = root_id;

|---a------a1 
|     |------a2

   ROOT_ID         ID NAME DESCRIPTIO
---------- ---------- ----- ----------
         0          1 a     aaa
         1          2 a1    aaa1
         1          3 a2    aaa2
         
select * from t2 start with id = 4 connect by prior id = root_id;

|---b------b1
      |------b2

   ROOT_ID         ID NAME DESCRIPTIO
---------- ---------- ----- ----------
         0          4 b     bbb
         4          5 b1    bbb1
         4          6 b2    bbb2
         
如果connect by prior中的prior被省略，则查询将不进行深层递归。
如：

select * from t2 start with root_id = 0 connect by id = root_id;

|---a
|    
|
|---b

   ROOT_ID         ID NAME DESCRIPTIO
---------- ---------- ----- ----------
         0          1 a     aaa
         0          4 b     bbb

如：         
select * from t2 start with id = 1 connect by id = root_id;

|---a

   ROOT_ID         ID NAME DESCRIPTIO
---------- ---------- ----- ----------
         0          1 a     aaa

 

原文地址：http://hi.baidu.com/learnfordba/blog/item/28fe4608fcb2112d6a60fbdb.html