请解释我所附的查询

Question

伙计们，

有人可以帮我解释一下下面的查询吗？

 SELECT   SOME_ID ROOT_CAT,
          CHILD_ID SOME_ID
 FROM    CHILD_TABLE
 START WITH SOME_ID IN (SELECT ROOT_ID FROM ROOT_TABLE)
 CONNECT BY NOCYCLE SOME_ID = PRIOR CHILD_ID;

Answer 1

这用于获取分层数据，例如 Parent - Child - Child of child 等。

start with -- 标识树中的所有 LEVEL=1 节点
connect by -- 描述如何从上面的父节点走到它们的子节点和 他们孩子的孩子。

一般语法是

select ... 
start with initial-condition 
connect by nocycle 
recurse-condition

connect by 构造建立递归。通过 SOME_ID= CHILD_ID 可以找到属于下一个较低层级的所有记录。

CHILD_ID 是在当前层级中找到的值

您的查询可以类似于：

 SELECT   SOME_ID ROOT_CAT,
          CHILD_ID SOME_ID
 FROM    CHILD_TABLE
 START WITH SOME_ID IN (SELECT ROOT_ID FROM ROOT_TABLE)
 CONNECT BY NOCYCLE SOME_ID = PRIOR CHILD_ID;

 SELECT  PARENT_ID  CURRENT_PARENT,
         CHILD_ID   PARENT_ID_CURRENT_CHILD
 FROM LIST_OF_CHILDREN
 START WITH PARENT_ID IN ( SELECT SOME_RANDOM_PARENT_ID FROM LIST_OF_PARENTS)
 CONNECT BY NOCYCLE  PARENT_ID = PRIOR CHILD_ID