MySQL合并来自多个表的列

Question

我正在尝试通过以下方式选择一些数据：

field:
+----------+------------+-----------+
|     id   |   room_id  | server_id |
+----------+------------+-----------+
|     1    |      34    |     0     |
|     2    |      34    |     0     |
|     3    |      35    |     1     |
+----------+------------+-----------+


user_position:
+----------+------------+-----------+
| user_id  | server_id  | position  |
+----------+------------+-----------+
|     11   |      0     |     2     |
|     17   |      1     |     25    |
|     19   |      0     |     28    |
+----------+------------+-----------+

room:
+----------+------------+-----------+
|     id   | server_id  | background|
+----------+------------+-----------+
|     34   |      0     |  #d91a1a  |
|     35   |      1     |  #f81b2a  |
|     36   |      0     |  #191b4a  |
+----------+------------+-----------+


RESULT:
(I hope I didn't mess it up)
+----------+------------+-----------+------------+------------+
|     id   | server_id  | background|  room_id   |  user_id   |
+----------+------------+-----------+------------+------------+
|     1    |      0     |  #d91a1a  |     34     |    null    |
|     2    |      0     |  #d91a1a  |     34     |     11     |   
|     3    |      1     |  #f81b2a  |     35     |    null    |
|    25    |      1     |    null   |    null    |     17     |
|    28    |      0     |    null   |    null    |     19     |
+----------+------------+-----------+------------+------------+

不幸的是，我无法编写正确的查询来实现此结果。我能得到的最好的结果是 field.id、user_position.position 和 field.room_id、user_position.room_id 列是分开的。我不知道如何将它们合并在一起。

有人可以帮帮我吗？

更新

好的，经过一番尝试，我得到了这个：

SELECT field.id, field.server_id, field.room_id, null AS user_id, room.background
FROM field
LEFT JOIN room ON room.id = field.room_id
WHERE field.server_id = 0
UNION
SELECT user_position.position, user_position.server_id, null, user_position.user_id, room.background
FROM user_position
LEFT JOIN room ON room.id = (SELECT field.room_id FROM field WHERE field.id = user_position.position)
WHERE user_position.server_id = 0

现在它正在工作我只是想问是否有更好的方法来实现相同的结果。还是您认为这个查询足够好？

Answer 1

有了给定的数据集，你可以试试下面 -

SELECT COALESCE(F.id, UP2.position) id
      ,COALESCE(F.server_id, UP2.server_id) server_id
      ,UP2.background
      ,F.room_id
      ,UP2.user_id
FROM field F
LEFT OUTER JOIN (SELECT UP.server_id, R.background, UP.position, UP.user_id
                 FROM (SELECT server_id, position, user_id, ROW_NUMBER()OVER(PARTITION BY server_id ORDER BY user_id) rn
                       FROM user_position) UP
                 JOIN (SELECT server_id, background, ROW_NUMBER()OVER(PARTITION BY server_id ORDER BY id) rn
                       FROM room) R ON UP.rn = R.rn
                                    AND UP.server_id = R.server_id) UP2 ON F.id = UP2.position
UNION
SELECT COALESCE(F.id, UP3.position) id
      ,COALESCE(F.server_id, UP3.server_id) server_id
      ,UP3.background
      ,F.room_id
      ,UP3.user_id
FROM field F
RIGHT OUTER JOIN (SELECT UP.server_id, R.background, UP.position, UP.user_id
                 FROM (SELECT server_id, position, user_id, ROW_NUMBER()OVER(PARTITION BY server_id ORDER BY user_id) rn
                       FROM user_position) UP
                 JOIN (SELECT server_id, background, ROW_NUMBER()OVER(PARTITION BY server_id ORDER BY id) rn
                       FROM room) R ON UP.rn = R.rn
                                    AND UP.server_id = R.server_id) UP3 ON F.id = UP3.position
ORDER BY id

Here 是演示。