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
例如：
数据库表结构如下：
1 2 3 4 5 6 7 8 9 10 11 12 | 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'); |
如图：
1.获取完整树：
1 2 | select * from t2;select * from t2 start with root_id = 0 connect by prior id = root_id; |
1 | select * from t2 start with id = 1 connect by prior id = root_id; |
1 | select * from t2 start with id = 4 connect by prior id = root_id; |
3.如果connect by prior中的prior被省略，则查询将不进行深层递归。
如：
1 | select * from t2 start with root_id = 0 connect by id = root_id; |
1 | select * from t2 start with id = 1 connect by id = root_id; |
1、start with id= 是定义起始节点（种子），可以是id也可以是root_id,定义为root_Id查询该节点下所有的树结构，定义为id（子节点）则查询指定的树
2、connect by prior ：prior的含义为先前，前一条记录。prior id=root_id 也就是前一条记录
的id等于当前记录的root_id(父id)
3、可以向下或者向上查找。。。。
4、level字段为oracle特有的层级字段，可以通过level字段查询指定的层级
select root_id,id,name,level from t2 where level=1
connect by prior id = root_id;
