MySQL 2 tables UNION - mysql

Question

I ask for your understanding because I can not speak English well.
There are 4 tables.
sensor (PK: sensor)
sensor | service
1 | 1
2 | 2
equipment (PK: id, FK: sensor)
id | equip | sensor |
1 | 8 | 1 |
2 | 8 | 1 |
3 | 8 | 2 |
4 | 7 | 2 |
A (PK: AUTO INCREMENT, UNIQUE: id, time, FK: id)
id | time | temperature
1 | 1027 | 30
1 | 1028 | 30
1 | 1029 | NULL
1 | 1030 | 60
1 | 1101 | 999
B (PK: AUTO INCREMENT, UNIQUE: id, time, FK: id)
id | time | temperature
2 | 1027 | 40
2 | 1029 | 50
2 | 1030 | NULL
2 | 1031 | 59
I want the following results.
time | A_temperature | B_temperature
1027 | 30 | 40
1028 | 30 | NULL
1029 | NULL | 50
1030 | 60 | NULL
1031 | NULL | 59
So I made the following query:
SELECT DISTINCT COALESCE(A.time, B.time) time, A.temperature AS A_temperature, B.temperature AS B_temperature
FROM equipment AS equip
JOIN sensor sen
ON sen.sensor = equip.sensor
AND sen.service = 1
JOIN A
ON A.id = equip.id
AND (A.time>= '1027' AND A.time<= '1031')
LEFT JOIN B
ON B.id = equip.id
AND (B.time>= '1027' AND B.time<= '1031')
AND B.time= A.time
WHERE equip.equip = 8
UNION
SELECT DISTINCT COALESCE(B.time, A.time) time, A.temperature AS A_temperature, B.temperature AS B_temperature
FROM equipment AS equip
JOIN sensor sen
ON sen.sensor = equip.sensor
AND sen.service = 1
LEFT JOIN B
ON B.id = equip.id
AND (B.time>= '1027' AND B.time<= '1031')
AND B.time= A.time
JOIN A
ON A.id = equip.id
AND (A.time>= '1027' AND A.time<= '1031')
WHERE equip.equip = 8
ORDER BY time ASC;
But I did not get the results I wanted.
time | A_temperature | B_temprature
1027 | 30 | NULL
1027 | NULL | 40
1028 | 30 | NULL
1028 | NULL | NULL
1029 | NULL | 50
....
After executing the above query, A.time and B.time are separated and the result is output. I want to combine these at the same time. If time is null, we want to put it in non-null time.

Answer 1

I think you can simplify the answer from your previous question by creating a subquery which just selects all the distinct time values from tables A and B. Then you can separately JOIN the A and B tables to it and GROUP BY the time values, using MAX to aggregate the temperatures since the values will be either valid or NULL, which MAX will ignore:
SELECT t.time, MAX(A.temperature) AS A_temperature, MAX(B.temperature) AS B_temperature
FROM equipment e
JOIN sensor s ON s.sensor = e.sensor AND s.service = 1
JOIN (SELECT time FROM A UNION SELECT time FROM B) t
LEFT JOIN A on A.id = e.id AND A.time = t.time
LEFT JOIN B on B.id = e.id AND B.time = t.time
WHERE t.time BETWEEN 1027 AND 1031
GROUP BY t.time
ORDER BY t.time
Output:
time A_temperature B_temperature
1027 30 40
1028 30
1029 50
1030 60
1031 59
Demo