Running sum from a point - sql-server

Question

I have a forecast of change that I need to add on to actuals.
Example:
Date Group Count ActForc
Nov-15 GrpA 10 A
Dec-15 GrpA 12 A
Jan-16 GrpA -1 F
Feb-16 GrpA 2 F
What I would like to see is:
Date Group Count
Nov-15 GrpA 10
Dec-15 GrpA 12
Jan-16 GrpA 11
Feb-16 GrpA 13
but all of the counting/running sum queries I have seen assume that I want the sections to be separate, and give me ways to create sums for each section, but essentially, I want to seed the sum for the second section with the final value from the first section, and continue from that point, without disturbing the values from the second section

Answer 1

If your forecasts are always in the end of the date range, you can also do this by using few window functions inside each other. Here is a running total calculated over a field that checks if the next row is 'F' then it takes count, otherwise 0. When that is then taken instead of count when the next row is F, it will contain the figure you want.
select
[date],
[group],
case when isnull(lead(ActForc) over (order by Date asc),ActForc) = 'F' then
sum(Count2) over (order by Date asc) else [Count] end,
[count],
ActForc
from (
select
[date],
[group],
case when isnull(lead(ActForc) over (order by Date asc),ActForc) = 'F' then [Count] else 0 end as Count2,
[count],
ActForc
from
table1
) X
This should perform better than any recursive CTEs / correlated subqueries because the data isn't read several times. If you have more groups, partitioning the window functions with the group should fix that.
Example in SQL Fiddle with few more months.

Answer 2

Try with a recursive cte.
First create a subquery to have a row_id
Then create the base case with rn = 1
And finally the recursion calculate each next level.
SQL Fiddle Demo
WITH addID as (
SELECT [Date], [Group], [Count], [ActForc],
ROW_NUMBER() OVER ( ORDER BY [DATE]) as rn
FROM myTable
), cte_name ( [Date], [Group], [Count], [level] ) AS
(
SELECT [Date], [Group], [Count], 1 as [level]
FROM addID
WHERE rn = 1
UNION ALL
SELECT A.[Date],
A.[Group],
CASE WHEN [ActForc] = 'F' THEN C.[Count] + A.[Count]
ELSE A.[Count]
END AS [Count],
C.[level] + 1
FROM addID A
INNER JOIN cte_name C
ON A.rn = C.[level] + 1
)
SELECT *
FROM cte_name
OUTPUT
| Date | Group | Count | level |
|----------------------------|-------|-------|-------|
| November, 01 2015 00:00:00 | GrpA | 10 | 1 |
| December, 01 2015 00:00:00 | GrpA | 12 | 2 |
| January, 01 2016 00:00:00 | GrpA | 11 | 3 |
| February, 01 2016 00:00:00 | GrpA | 13 | 4 |