Question

I have asked a similar question previously, but posting a new one as I do not want to confuse other
members, and there is an additional column.

What I am looking for is to update the column ItemActual. This needs to be updated with the difference with ItemValue for the latest CurrentTime for the same StartTime if any.

If there is no other entry for the same StartTime other than the current row, it needs to be the ItemValue itself. The comparisons are only to be done for items with the same name.

For eg, Rownum 283, ItemActual = 347013 (since there is no other row with same StartTime).
This applies to row 235 as well, i.e. ItemActual = 1086054.00

For row 190, this needs to be 664031.00 - 533023.00 (comparing with row 145) = 131008
But for row 10, this will be 532023.00, since there is no earlier entry of same item with same StartTime.

Rownum  Name  ItemValue      CurrentTime        StartTime
 283    ABC     347013.00     3/05/2012 16:01   29/04/2012 6:29
 235    ABC    1086054.00    26/03/2012 14:05    7/03/2012 21:18
 190    ABC     664031.00    13/02/2012 13:42   29/01/2012 6:39
 145    ABC     533023.00     7/02/2012 14:01   29/01/2012 6:39
 100    ABC     532023.00     7/02/2012 13:33   29/01/2012 6:39
  55    ABC     532023.00     7/02/2012 12:52   29/01/2012 6:39
  10    ABC     532023.00     7/02/2012 12:51   29/01/2012 6:39
 310    DEF     351012.00     3/05/2012 16:01   29/04/2012 6:29
 261    DEF    1339066.00    26/03/2012 14:05    7/03/2012 21:18
 215    DEF     785034.00    13/02/2012 13:42   29/01/2012 6:39
 170    DEF     620026.00     7/02/2012 14:01   29/01/2012 6:39

Answer 1

You can try something like this:

;WITH PartData AS 
(     
    SELECT
        RowNum, Name, ItemValue, CurrentTime, StartTime,
        RX = ROW_NUMBER() OVER(PARTITION BY Name,StartTime ORDER BY CurrentTime DESC)
    FROM dbo.YourTable
)
SELECT     
    p1.RowNum, p1.ItemValue, p1.CurrentTime, p1.StartTime, 
    RowNumComparedTo = p2.RowNum, 
    ItemActual = CASE 
                    WHEN p2.RX IS NULL THEN p1.ItemValue
                    ELSE p1.ItemValue - p2.ItemValue
                 END
FROM PartData p1
LEFT OUTER JOIN PartData p2 ON p1.StartTime = p2.StartTime 
                               AND p1.Name = p2.Name 
                               AND p2.RX = p1.RX + 1

I get an output something like this:

RowNum  ItemValue   CurrentTime       StartTime        RowNumComparedTo  ItemActual
 190     664031.00  2012-02-13 13:42  2012-01-29 06:39    145            131008.00
 145     533023.00  2012-02-07 14:01  2012-01-29 06:39    100              1000.00
 100     532023.00  2012-02-07 13:33  2012-01-29 06:39     55                 0.00
  55     532023.00  2012-02-07 12:52  2012-01-29 06:39     10                 0.00
  10     532023.00  2012-02-07 12:51  2012-01-29 06:39    NULL           532023.00
 215     785034.00  2012-02-13 13:42  2012-01-29 06:39    170            165008.00
 170     620026.00  2012-02-07 14:01  2012-01-29 06:39    NULL           620026.00
 235    1086054.00  2012-05-03 14:05  2012-03-07 21:18    NULL          1086054.00
 261    1339066.00  2012-03-26 14:05  2012-03-07 21:18    NULL          1339066.00
 283     347013.00  2012-05-03 16:01  2012-04-29 06:29    NULL           347013.00
 310     351012.00  2012-05-03 16:01  2012-04-29 06:29    NULL           351012.00

The solution basically does this:

• it creates a CTE (Common Table Expression) and “partitions” your data by Name,StartTime and orders these rows by CurrentTime DESC – so the most recent entry for each Name,StartTime group will get an RX (Row indeX) of 1

• it then joins that CTE against itself, shifted by one RX – so I’m comparing RX = 1 to RX = 2 (if present) etc.

• if a “shifted” row is present, the difference in the ItemValue values is returned as ItemActual – otherwise the ItemValue from the main row is returned

I hope this solves your problem