SQL Server Row by Row operation

Question

I am trying to do some operation in column 'Increment' based on previous row record in column 'Value'

e.g.

row_num| Period | Measure | Decay
1      | Jan 08 | 10      | 
2      | Feb 08 | 18      | 
3      | Mar 08 | 7       | 
4      | Apr 08 | 67      | 

i would like to update column 'Decay' based on a formula

row_num| Period | Measure| Decay
1      | Jan 08 | 10     | = 10             -> first value in 'Measures'
2      | Feb 08 | 18     | = 10*0.5+18 = 23 -> previous decay record *0.5 + current measure
3      | Mar 08 | 7      | = 23*0.5+7  = 18.5
4      | Apr 08 | 67     | = 18.5*0.5+67 = 76.25

would cursor be applicable here? how would the syntax be like? thank you

Answer 1

Here's a running example using a recursive CTE (also note that the arithmetic in your example is incorrect):

-- SO3192010

DECLARE @t AS TABLE (row_num int NOT NULL, Period varchar(6) NOT NULL, Measure float NOT NULL)
INSERT INTO @t VALUES (1, 'Jan 08', 10)
    ,(2, 'Feb 08', 18)
    ,(3, 'Mar 08', 7)
    ,(4, 'Apr 08', 67)

;WITH r AS (
    SELECT t.*, Measure AS Decay
    FROM @t AS t
    WHERE t.row_num = 1

    UNION ALL

    SELECT t.*, r.Decay * 0.5 + t.Measure AS Decay
    FROM r
    INNER JOIN @t AS t
        ON t.row_num = r.row_num + 1
)
SELECT *
FROM r
ORDER BY row_num​