SQL Server Row by Row operation

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

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​
```

```UPDATE
tbl_test
SET
col_name = 'xyq'
FROM
(
SELECT
*,
ROW_NUMBER() OVER (ORDER BY name) AS rno
FROM
tbl_test
)tbl_test
WHERE
rno = 3

--rno is the rownumber that u want to update
```