MySQL get difference between max and min values for select rows

Question

I want a query that gets the difference between the max and min values grouped by day. My attempt does not work:

SELECT date(`date`), 
   (max(value) - min(value)) as value,
FROM `sub_meter_data` 
where date(sub_meter_data.date) > '2012-10-01' 
  and sub_meterID in('58984','58985','58986','58987')
group by date(`date`);

Each sub_meter has a value that might be >3000, but only differs by < 10 per day. I want the difference, ie a result <10. With the query above I get results >3000.

This query below, just selects one meter, and give the correct results, the max (17531), the min (17523), and the difference (8).

SELECT date(sub_meter_data.date) as date,
   max(value) as max_meter,
   min(value) as min_meter, 
   max(value) - min(value) as diff,
FROM `sub_meter_data`
where date(sub_meter_data.date) > '2012-10-01' 
  and sub_meterID in('57636')
  group by date(sub_meter_data.date)

But adding another meter into the in clause, give a bad result, the max is 17531, and the min is 3021, the diff is 14510. But I want the diff for each meter, then summed together.

SELECT date(sub_meter_data.date) as date,
   max(value) as max_meter,
   min(value) as min_meter, 
   max(value) - min(value) as diff,
   FROM `sub_meter_data`
where date(sub_meter_data.date) > '2012-10-01' 
  and sub_meterID in('57636', '57628')
  group by date(sub_meter_data.date)

Another attempt I've tried is:

SELECT date(sub_meter_data.date) as date,
   sum(CASE WHEN sub_meterID = '57628' OR sub_meterID = '57636' THEN (max(value) -     min(value)) ELSE 0 END) as value
   FROM `sub_meter_data`
where date(sub_meter_data.date) > '2012-10-01' 

Answer 1

The query is only grouping by day (date), but you want to group also by meter, so you need to add that into your group by:

select sub_meterID, date(`date`) as day, max(value) - min(value) as value
from `sub_meter_data`
where date(`date`) > '2012-10-01'
    and sub_meterID in ('58984','58985','58986','58987')
group by sub_meterID, date(`date`);

Then if you want to sum the differences by day you can do:

select day, sum(diff) as total_diff
from (
    select sub_meterID, date(`date`) as day, max(value) - min(value) as diff
    from `sub_meter_data`
    where date(`date`) > '2012-10-01'
       and sub_meterID in ('58984','58985','58986','58987')
    group by sub_meterID, date(`date`)
    ) a
group by day

Or if you want to sum by meter:

select sub_meterID, sum(diff) as total_diff
from (
    select sub_meterID, date(`date`) as day, max(value) - min(value) as diff
    from `sub_meter_data`
    where date(`date`) > '2012-10-01'
       and sub_meterID in ('58984','58985','58986','58987')
    group by sub_meterID, date(`date`)
    ) a
group by sub_meterID