MySQL merge columns from multiple tables - mysql

Question

i'm trying to select some data in the following way:
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 |
+----------+------------+-----------+------------+------------+
Unfortunately i couldn't write the right query to achieve this result. The best I could get was that the field.id, user_position.position and field.room_id, user_position.room_id columns were separated. I have no idea how to merge them together.
Can somebody help me?
UPDATE
OK, so after some trying I got this:
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
Now it is working I just want to ask if there isn't a better way to achieve the same result. Or do you think this query is good enough?

Answer 1

With the given set of data, you may try below -
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 is the demo.