查询以获取两个表中的最大值mysql

Question

I need to get the max value in two tables and display in four columns differents. Celsius | Hum | fecha_temp | fecha_hum

Table: temperaturas
Columns:
 id_temp int(11) AI PK 
 celsius_temp decimal(10,0) 
 fah_temp decimal(10,0) 
 fecha_temp datetime

Table: humedad
Columns:
 id_hum int(11) AI PK 
 hum_hum float 
 fecha_hum datetime

I tried this query but doesn't work

select t.celsius_temp as celsius, h.hum_hum, t.fecha_temp, h.fecha_hum from 
temperaturas t
inner join humedad h on h.hum_hum <=(select max(h.hum_hum) from humedad h)
where t.celsius_temp<=(select max(t.celsius_temp) from temperaturas t) and 
t.fecha_temp between '2017-12-01' and '2017-12-05'
order by t.celsius_temp, h.hum_hum desc limit 1;

Thanks so much

Answer 1

The max value of each table in the period is:

SET @start_date = '2017-12-01';
SET @end_date = '2017-12-01';
SELECT MAX(t.celsius_temp) INTO @tmax FROM temperaturas t 
  WHERE t.fecha_temp BETWEEN @start_date AND @end_date;
SELECT MAX(h.hum_hum) INTO @hmax FROM humedad h 
  WHERE t.fecha_hum between @start_date AND @end_date;

You could get them both into one table doing:

SELECT @tmax, @hmax;

If you want to know the dates where the max temperature or max humidity is reached it's a bit trickier as you may have multiple dates with the same values. You could write this as a single query but I'd rather use the above queries and then run:

SELECT * from temperaturas t where t.celsius_temp = @maxt;
SELECT * from humedad h where h.hum_hum = @maxh;

Remember, this could have multiple rows if the temperature is the same on multiple dates. There is no simple sensible way how would would join this into one table.

If you have multiple measurements per day and are looking for the top temperature/humidity on each given day, then you want to use the Group By function. You can then join on the date() functio of the timestamp like this:

SELECT coalesce(date(t.fecha_temp), date(h.fecha_hum)), 
  MAX(t.celsius_temp) as celsius,
  MAX(h.hum_hum) as humibity
FROM temperaturas t 
OUTER JOIN humedad h on date(t.fecha_temp) = date(h.fecha_hum)
WHERE coalesce(date(t.fecha_temp), date(h.fecha_hum)) 
  between @start_date and @end_date
GROUP BY coalesce(date(t.fecha_temp), date(h.fecha_hum))
ORDER BY coalesce(date(t.fecha_temp), date(h.fecha_hum)) 

The coalesce() function here is necessary because you may have dates for which only one table may have data.

This join is not efficient if you have a lot of data, in which case you may want to look into creating temporary tables with the group function results and then join just on the single row per date.