SQL 查询返回不正确的值

Question

I have a MariaDB weather database that is updated every 15 minutes with current conditions. I am trying to query it to return the maximum temperature for a specific month. When I query it with "Select temp,datetime from table" I get a listing of all temps and the time as recorded. I may get 3-4 records of the same temp with 15min time intervals over an hour or so. To get a specific month, I tried "SELECT temp,logdatetime FROM Monthly WHERE logdatetime like '2020-01%' ORDER BY temp desc" I get what I expect. (Datetime format is YYYY-MM-DD HH:mm:ss)

temp	logDateTime
40.2	2020-01-04 16:15:00
40.1	2020-01-04 15:30:00
40.1	2020-01-04 17:00:00
40.1	2020-01-04 17:30:00
40.1	2020-01-31 16:15:00
40.0	2020-01-04 16:45:00
40.0	2020-01-31 16:00:00

If I try "SELECT max(temp),logdatetime FROM Monthly WHERE logdatetime like '2020-01%'", results are;

temp	logdatetime
40.2	2020-01-01 00:00:00

In this example the date has returned incorrectly as Jan 1 rather than Jan 4, and the time is missing. There is only one entry for 40.2 so shouldn't it return the time. I understand if there are multiple temps, as for 40.1, there may be a problem returning the four dates.

I've also tried a nested select statement as below, with the same result. SELECT max(a.temp), LogDateTime FROM (SELECT temp, logdatetime FROM Monthly WHERE logdatetime like '2020-01%') AS a

Any help greatly appreciated. Peter

Answer 1

Use

SELECT *
FROM logdatetime 
WHERE logdatetime BETWEEN '2020-01-01 00:00:00' AND '2020-01-31 23:59:59'
ORDER BY temp DESC
LIMIT 1

This query selects a row with maximal temperature in Jan, 2020.

If there exists a lot of rows with the same maximal temperature value then this query will return one of them (and in general you cannot predict what row from all possible ones will be returned). If you need in some definite (for example, the most last) then you may expand sorting expression (for example, till ORDER BY temp DESC, logdatetime DESC ).

If you need all rows with maximal temperature, then you must firstly get this maximal value in subquery then use it in the query for rows filtering. Or, in MySQL 8+ version, you may use RANK() function in CTE.

Answer 2

Akina, That worked great. Many thanks for your help. Peter