sql - MySQL - Subtracting value from previous row, group by

Question

I need to have the consumption value base on previous one by SN number. This is my data:

TABLE EnergyLog

SN     Date                 Value
2380   2012-10-30 00:15:51  21.01
2380   2012-10-31 00:31:03  22.04
2380   2012-11-01 00:16:02  22.65
2380   2012-11-02 00:15:32  23.11
20100  2012-10-30 00:15:38  35.21
20100  2012-10-31 00:15:48  37.07
20100  2012-11-01 00:15:49  38.17
20100  2012-11-02 00:15:19  38.97
20103  2012-10-30 10:27:34  57.98
20103  2012-10-31 12:24:42  60.83

This is the result I need:

SN      Date                 Value  consumption
2380    2012-10-30 00:15:51  21.01  0
2380    2012-10-31 00:31:03  22.04  1.03
2380    2012-11-01 00:16:02  22.65  0.61
2380    2012-11-02 00:15:32  23.11  0.46
20100   2012-10-30 00:15:38  35.21  0
20100   2012-10-31 00:15:48  37.07  1.86
20100   2012-11-01 00:15:49  38.17  1.1
20100   2012-11-02 00:15:19  38.97  0.8
20103   2012-10-30 10:27:34  57.98  0
20103   2012-10-31 12:24:42  60.83  2.85

Answer 1

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