MySQL - Subtracting value from previous row, group by

Working with MySQL variables is great, its like inline program variable assignments. First, the FROM clause "declares" the @ variables for you, defaulting to blank. Then query the records in the expected order you want them. It makes a single pass through the data instead of via repeated subqueries which can be time intensive.

For each row read, compare the @lastSN with the SN of the current record. If different, always return 0. If it IS the same, compute the simple difference. Only AFTER that compare is done, set the @lastSN and @lastValue equal to that of the current record for the next records comparison.

select
      EL.SN,
      EL.Date,
      EL.Value, --remove duplicate alias
      if( @lastSN = EL.SN, EL.Value - @lastValue, 0000.00 ) as Consumption,
      @lastSN := EL.SN,
      @lastValue := EL.Value
   from
      EnergyLog EL,
      ( select @lastSN := 0,
               @lastValue := 0 ) SQLVars
   order by
      EL.SN,
      EL.Date

This should do the trick:

SELECT l.sn,
       l.date, 
       l.value,
       l.value - (SELECT value 
                  FROM energylog x
                  WHERE x.date < l.date
                  AND x.sn = l.sn
                  ORDER BY date DESC
                  LIMIT 1) consumption
FROM energylog l;

See SQLFiddle: http://sqlfiddle.com/#!2/b9eb1/8

A near universal solution is to join the data on to itself, to find the previous record, by including a correlated sub-query in the join condition...

SELECT
  ThisLog.*,
  COALESCE(ThisLog.Value - PrevLog.Value, 0) AS consumption
FROM
  EnergyLog    AS ThisLog
LEFT JOIN
  EnergyLog    AS PrevLog
    ON  PrevLog.SN   = ThisLog.SN
    AND PrevLog.Date = (SELECT MAX(Date)
                          FROM EnergyLog
                         WHERE SN   = ThisLog.SN
                           AND Date < ThisLog.Date)

This performs best with one index covering both (SN, Date).

You can join two rows of the same table like this:

    SELECT this.*, prev.*
      FROM tbl this
INNER JOIN tbl prev ON prev.id =
           ( 
               SELECT max(t.id) 
               FROM tbl t  
               WHERE t.id < this.id
           )     
     WHERE ...

So your case will look like:

    SELECT this.SN, this.Date, this.Value, (this.Value - prev.Value) AS consumption
      FROM EnergyLog this
INNER JOIN EnergyLog prev ON prev.Date =
           ( 
               SELECT max(t.Date) 
               FROM EnergyLog t  
               WHERE t.Date < this.Date
           )