oracle

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

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;

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;