How to group by day/month/year on a subquery

Question

I have a simple table like this (in real x-thousand values):

On this table I'm doing a query to get the differences (which works fine):

select
  EL.sensor_id,
  EL.value_id,
  EL.time,
  EL.value,
  if( @lastSN = EL.sensor_id, EL.value - @lastValue, 0000.00 ) as diff,
  @lastSN := EL.sensor_id,
  @lastValue := EL.value
from
      data_test_debug EL,
      ( select @lastSN := 0,
               @lastValue := 0 ) SQLVars
   where EL.value_id = "gas"
   order by
      EL.sensor_id,
      EL.time

The result looks like this:

Now I have a problem because I have to group the diff data.

by day or month or week or year (for representing the data in charts)

I tried something like this:

select MONTHNAME(EL.time), SUM(EL.diff)
FROM data_test_debug EL,

(select
      EL.sensor_id,
      EL.value_id,
      EL.time,
      EL.value,
      if( @lastSN = EL.sensor_id, EL.value - @lastValue, 0000.00 ) as diff,
      @lastSN := EL.sensor_id,
      @lastValue := EL.value
   from
      data_test_debug EL,
      ( select @lastSN := 0,
               @lastValue := 0 ) SQLVars
   where EL.value_id = "gas"
   order by
      EL.sensor_id,
      EL.time) AS t
GROUP BY YEAR(EL.time), MONTH(EL.time)

but had no look to get results..

The data is hourly collected therefore grouping is needed.

Answer 1

Don't join the table again. Apply the grouping directly to the subquery:

SELECT YEAR(time), MONTHNAME(time), SUM(diff)
FROM (select EL.sensor_id,
             EL.value_id,
             EL.time,
             EL.value,
             if(@lastSN = EL.sensor_id, EL.value - @lastValue, 0.0) as diff,
             @lastSN := EL.sensor_id,
             @lastValue := EL.value
      FROM data_test_debug EL, (SELECT @lastSN := 0,
                                @lastValue := 0) SQLVars
      WHERE EL.value_id = "gas"
      ORDER BY EL.sensor_id, EL.time
     ) AS t
GROUP BY YEAR(time), MONTH(time)

It can be greatly simplified by removing all those fields that you're not using, but you can do that after you get it working.