Accumulative Update for previous records - sql-server

Question

I have table that shows these information
Month NewClients OnHoldClients
5-2017 10 2
6-2017 16 4
7-2017 11 1
8-2017 15 6
9-2017 18 7
I am trying to find the accumulative total for each month
which is
(NewClients - OnHoldClients) + Previous Month Total
Something like this
Month NewClients OnHoldClients Total
5-2017 10 2 8
6-2017 16 4 20
7-2017 11 1 30
8-2017 15 6 39
9-2017 18 7 50
the query i tried to build was something like this but I think should be an easier way to do that
UPDATE MyTable
SET Total = (SELECT TOP 1 Total FROM MyTable B WHERE B.Month < A.Month) + NewClients - OnHoldClients
FROM MyTable A

Answer 1

Before we begin, note the mere fact that you're facing such calculative problem is a symptom that maybe you don't have the best possible design. Normally for this purpose calculated values are being stored along the way as the records are inserted. So i'd say you'd better have a total field to begin with and calculate it as records amass.
Now let's get down to the problem at hand. i composed a query which does that nicely but it's a bit verbose due to recursive nature of the problem. However, it yields the exact expected result:
DECLARE #dmin AS date = (SELECT min(mt.[Month]) from dbo.MyTable mt);
;WITH cte(_Month, _Total) AS (
SELECT mt.[Month] AS _Month, (mt.NewClients - mt.OnHoldClients) AS _Total
FROM dbo.MyTable mt
WHERE mt.[Month] = #dmin
UNION ALL
SELECT mt.[Month] AS _Month, ((mt.NewClients - mt.OnHoldClients) + ccc._Total) AS _Total
FROM dbo.MyTable mt
CROSS APPLY (SELECT cc._Total FROM (SELECT c._Total,
CAST((row_number() OVER (ORDER BY c._Month DESC)) AS int) as _Rank
FROM cte c WHERE c._Month < mt.[Month]) as cc
WHERE cc._Rank = 1) AS ccc
WHERE mt.[Month] > #dmin
)
SELECT c._Month, max(c._Total) AS Total
FROM cte c
GROUP BY c._Month
It is a recursive CTE structure that goes about each record all along the way to the initial month and adds up to the final Total value. This query only includes Month and Total fields but you can easily add the other 2 to the list of projection.

Answer 2

Try this
;WITH CTE([Month],NewClients,OnHoldClients)
AS
(
SELECT '5-2017',10,2 UNION ALL
SELECT '6-2017',16,4 UNION ALL
SELECT '7-2017',11,1 UNION ALL
SELECT '8-2017',15,6 UNION ALL
SELECT '9-2017',18,7
)
SELECT [Month],
NewClients,
OnHoldClients,
SUM(MonthTotal)OVER( ORDER BY [Month]) AS Total
FROM
(
SELECT [Month],
NewClients,
OnHoldClients,
SUM(NewClients-OnHoldClients)OVER(PArtition by [Month] Order by [Month]) AS MonthTotal
FROM CTE
)dt
Result,Demo:http://rextester.com/DKLG54359
Month NewClients OnHoldClients Total
--------------------------------------------
5-2017 10 2 8
6-2017 16 4 20
7-2017 11 1 30
8-2017 15 6 39
9-2017 18 7 50