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今天工作看同事代码用到了start with...connect by prior，不知道什么意思，查询了一些资料，以下是比较好理解的。
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
where
start with
connect by ;
：过滤条件，用于对返回的所有记录进行过滤。
：查询结果重起始根结点的限定条件。
：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：