从另一个 table 获取 Child 的属性

Question

我准备了一份FIDDLE HERE

我有一个 parent table 如下所示。

+----------+------------+
| material | Attachment |
+----------+------------+
| 101      | 1          |
| 102      | 2          |
| 201      | 4          |
| 202      | 4          |
| 301      | 2          |
+----------+------------+

还有如下关系table

+--------+-------+
| parent | child |
+--------+-------+
| 101    | 201   |
| 101    | 202   |
| 101    | 204   |
| 101    | 205   |
| 102    | 301   |
| 102    | 302   |
+--------+-------+

我正在尝试获取以下格式的 child 附件。预期输出。

+--------+-------+------------+
| parent | child | Attachment |
+--------+-------+------------+
| 101    | 201   | 4          |
| 101    | 202   | 4          |
| 101    | 204   | Child NA   |
| 101    | 205   | Child NA   |
| 102    | 301   | 2          |
| 102    | 302   | Child NA   |
+--------+-------+------------+

我试过这个查询。但是我得到的附件是 Parent 而不是 child.

select c.parent,c.child,Attachment from parent p
join child c
on p.material=c.parent

下面。

+--------+-------+------------+
| parent | child | Attachment |
+--------+-------+------------+
| 101    | 201   | 1          |
| 101    | 202   | 1          |
| 101    | 204   | 1          |
| 101    | 205   | 1          |
| 102    | 301   | 2          |
| 102    | 302   | 2          |
+--------+-------+------------+

Answer 1

我认为这是一个 left join:

select r.parent, r.child, p.attachment
from relation r left join
     parent p
     on p.material = r.child;

这会产生 NULL 而不是 'Child NA'。

Here 是一个 db<>fiddle.

Answer 2

根据您的评论，您可以这样写：

   select c.parent, 
          c.child, 
          p.attachment
   from child c 
   left join parent p on p.material = c.child 
                         and 
                         c.parent IN (select material from parent);

从另一个 table 获取 Child 的属性

Get attribute of Child from another table

sql

sql-server

hierarchy

sql-server-2012