Earliest time of daily maximum values - mysql

Question

I have a table that logs weather data variables by datetime like this:
|------------------|------------| ----
| LogDateTime | Temp | ...
+------------------|------------| ----
| 2020-01-01 00:00 | 20.1 | ...
| 2020-01-01 00:05 | 20.1 | ...
| 2020-01-01 00:10 | 19.9 | ...
| 2020-01-01 00:15 | 19.8 | ...
---------------------------------------
From that table I want to return the earliest time of the maximum temperature for each day like this (just the time portion of the datetime value):
|------------|----------------------
| LogDate | LogTime| MaxTemp
+---------------------|--------------
| 2020-01-01 | 14:00 | 24.5
| 2020-01-02 | 15:12 | 23.2
| 2020-01-03 | 10:12 | 25.1
| 2020-01-04 | 12:14 | 28.8
--------------------------------
The query I have to return this so far is the below, but it returns the earliest temperature for each day instead of the earliest occurrence of the maximum temperature for each day
SELECT TIME(a.LogDateTime), a.Temp
FROM Monthly a
INNER JOIN (
SELECT TIME(LogDateTime), LogDateTime, MAX(Temp) Temp
FROM Monthly
GROUP BY LogDateTime
) b ON a.LogDateTime = b.LogDateTime AND a.Temp= b.Temp
GROUP BY DATE(a.LogDateTime)
I then want to use that query to update a table of one row per day that summarises the minimum and maximum values with a query something like this but update the time rather than the actual maximum temperature:
UPDATE Dayfile AS d
JOIN (
SELECT DATE(LogDateTime) AS date, MAX(Temp) AS Temps
FROM Monthly
GROUP BY date
) AS m ON DATE(d.LogDate) = m.date
SET d.MaxTemp = m.Temps

Answer 1

Your version of MariaDB supports window functions, so use ROW_NUMBER():
select LogDateTime, Temp
from (
select *,
row_number() over (partition by date(LogDateTime) order by Temp desc, LogDateTime) rn
from Monthly
) t
where t.rn = 1
See a simplified demo.
Use it to update Dayfile like this:
update Dayfile d
inner join (
select LogDateTime, Temp
from (
select *,
row_number() over (partition by date(LogDateTime) order by Temp desc, LogDateTime) rn
from Monthly
) t
where t.rn = 1
) m on date(d.LogDate) = m.date
set d.MaxTemp = m.Temp