在不同關系上加入同一張桌子

Question

我有3張桌子。 用戶，對象和項目。 用戶可以有許多對象，而對象可以有許多項目。 此外，用戶可以有許多項目。 我正在為這些表做報告。

我的結果應如下所示：

+------------+----------------+-----------------+
| username   | Object         | Item            |
+------------+----------------+-----------------+
|  1001      | Object_name1   | Item_name1      |
|  1001      | NULL           | Item_name2      |
|  1001      | NULL           | Item_name3      |
|  1001      | NULL           | Item_name4      |
|  1001      | NULL           | Item_name5      |
|  1002      | NULL           | Item_name6      |
+------------+----------------+-----------------+
and so on..

我目前查詢得到的最好的結果是：

SELECT u.username, o.name as object, i.name as item, i1.name as item_1
FROM users as u
LEFT JOIN object_user as ou on ou.user_id = u.id
LEFT JOIN objects as o on o.id = ou.object_id
LEFT JOIN object_items as oi on oi.object_id = o.id
LEFT JOIN items as i on i.id = oi.item_id
LEFT JOIN item_user as iu on iu.user_id = u.id
LEFT JOIN items as i1 on i1 = iu.item_id

+------------+----------------+-----------------+-----------------+
| username   | Object         | Item            | Item_1          |
+------------+----------------+-----------------+-----------------+
|  1001      | Object_name1   | Item_name1      | Item_name2      |
|  1001      | Object_name1   | Item_name1      | Item_name3      |
|  1001      | Object_name1   | Item_name1      | Item_name4      |
|  1001      | Object_name1   | Item_name1      | Item_name5      |
|  1002      | NULL           | NULL            | Item_name6      |
+------------+----------------+-----------------+-----------------+

我的架構： 如您所見，這是不正確的。 有什么建議怎么解決呢？

Answer 1

您可以對項目使用兩次連接，一次用於對象相關的項目，另一次用於用戶實現的項目

SELECT u.username, o.name as object, oi.name as oi_item_name, ui.name as ui_item_name
FROM users u
LEFT JOIN objects o on o.id = u.object_id 
LEFT JOIN items  oi on o.item_id  = oi.id
LEFT JOIN items ui on u.item_id  = ui.id