[FIXED] How to get difference each record value from previous record

Issue

Please consider this Table:

Year     Month    Code     Value
--------------------------------
2011      1         1       100
2011      2         1       200
2011      3         1       250
2011      1         2       70
2011      2         2       50
2011      3         2       80

I want to achieve this result:

Code    FromYearMonth      ToYearMonth    Difference
----------------------------------------------------
 1       2011-1               2011-2        100   <----200 - 100
 1       2011-2               2011-3        50    <----250 - 200
 2       2011-1               2011-2        -20   <----50 - 70
 2       2011-2               2011-3        30    <----80 - 50

How can I create this result with LINQ. Thanks

Solution

You need to join datasource with itself to get all pairs of (monthStart, monthEnd) records, then calculate difference:

from monthStart in dataSource
join monthEnd in dataSource
on new { monthStart.Code, date = monthStart.Year * 12 + monthStart.Month }
equals new { monthEnd.Code, date = monthEnd.Year * 12 + monthEnd.Month - 1}
select new {
  monthStart.Code,
  FromYear = monthStart.Year,
  FromMonth = monthStart.Month,
  ToYear = monthEnd.Year,
  ToMonth = monthEnd.Month,
  Difference = monthEnd.Value - monthStart.Value
}

Answered By - pakeha_by

Answer Checked By - Marilyn (PHPFixing Volunteer)