PHP，MySQL。 最低和最高价值的日，月和年

Question

Edit for clarification:

This is my table:

ID      Datetime            Date        Temp
6043    2016-05-24 10:20:00 2016-05-24  19.3
6042    2016-05-24 10:15:00 2016-05-24  19.1
6041    2016-05-24 10:10:00 2016-05-24  19.1
6040    2016-05-24 10:05:00 2016-05-24  19.1

What I'm trying to do is to select the row with the highest "Temp".

This is my query:

SELECT max(temp) as maxtemp FROM weatherdata WHERE day(date) = 24

The query only gives me the maxtemp, how can I get the ID and Datetime?

Goal is to get the max-value for each day in my table (WHERE day(date) = xx)

Kind Regards

Answer 1

It looks to me like you are trying to get your entire weatherdata observation row for the observation that contains the highest temperature in any given month.

This takes two steps, as you have realized. The first step: find the highest temperature observation in each month. You do that like so:

              SELECT MAX(temp) temp, LAST_DAY(`datetime`) month_ending
                FROM weatherdata
               GROUP BY LAST_DAY(`datetime`)

The trick here is to use LAST_DAY() to figure out in which month each observation occurs. It takes any date, datetime, or timestamp item and figures out the date of the last day of its month. This query gives you a result set containing one temp per month, for the highest temp in the month. Try it. Make sure it works for you.

Next, you should join that to your detailed observation table, like so:

 SELECT w.*
   FROM weatherdata w
   JOIN (
              SELECT MAX(temp) temp, LAST_DAY(`datetime`) month_ending
                FROM weatherdata
               GROUP BY LAST_DAY(`datetime`)
        ) m   ON w.temp = m.temp
             AND LAST_DAY(w.`datetime`) = m.month_ending
  ORDER BY w.`datetime`

This will give you the detailed observations. Notice that if the highest temperature occurred on more than one observation in any given month, you'll get them all.

Finally, if you only want some months, put this WHERE clause right before the ORDER BY clause

   WHERE `datetime` >= '2016-04-01 
     AND `datetime` <  '2016-05-01  -- to get April 2016 

Pro tip: don't use names of datatypes, like datetime , as column names. It makes it hard to troubeshoot your database.