oracle的connect by 什么意思

Question

见下面几个语句 （请问为什么第三个语句插入的数据只有一条， where 和 connect by的区别）：

create table t1 as select rownum as id, rownum+1 as id2 from dual connect by level<=5;
select * from t1

create table t2 as select rownum as id, rownum+1 as id2 from dual connect by rownum <=5;
select * from t2

create table t3 as select rownum as id, rownum+1 as id2 from dual where rownum <=5;
select * from t3

Answer 1

connect by 是oracle中特殊的递归查询的语法，多用于组织结构遍历、菜单树遍历等场景，如下面的一个功能菜单表，pid表示上级菜单的id，0表示无上级菜单，如果想查出顶级菜单下的所有菜单，可以用下面的写法：

id pid 
1 0
2 1
3 1
4 2
5 3

select *
from t
start with pid = 0
connect by pid = prior id

你给的例子中，是connect by的一个特殊用法，用来产生指定行的一个虚拟表。

Answer 2

select ... from ... connect by ... 递归遍历查询