oracle 树

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：

select * from tableName start with childid =1 connect by prior  childid = parentid

prior 方为基准方，在child侧为向下查找，反之则向上