Oracle Connect By Prior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：

　　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');

　　获取完整树：

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

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

    select * from t2 start with id = 4 connect by prior id = root_id;

    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;



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