SQL - How to Multiply Prior Row value to Current Row - sql

Question

Can this be done and if so what is the best approach. I tried Over(Partition) but cannot figure out how to add the prior columns. Should I use IF BEGIN ENDS and just walk through it row by row for running balance? Any assistance on initial direction would be appreciated.
CREATE TABLE [dbo].[PStatement](
[ID] [int],
[Source] [nvarchar](255),
[PaymentNumber] [int],
[PaymentAmt] [money],
[RBalance] [money],
[CBalance] [money],
[AllocationPercent] [float]
)
With the following values:
INSERT INTO PStatement (ID, [Source], PaymentNumber, PaymentAmt, RBalance, CBalance, AllocationPercent)
VALUES (1, N'SR', 1, 0, 7500, 10000, .75)
,(2, N'ER', 1, 0, 2500, 10000, .25)
,(3, N'SR', 2, 50, 0, 11000, 0)
,(4, N'ER', 2, 75, 0, 11000, .0)
,(5, N'SR', 3, 50, 0, 12000, 0)
,(6, N'ER', 3, 75, 0, 12000, .0)
I need to take the CBalance from the prior row, subtract the sum of the current rows PaymentAmt and multiple that amount by the prior rows AllocationPercentage and then add back the PaymentAmt based on PaymentNumber and Source.
So Row 3 RBalance is $11000 - ($50 + $75) (from current row) * .75 (from prior row) + 50 (from current row), AllocationPercent is RBalance/CBalance.
So Row 4 is $11000 - ($50 + $75) * .25 + 75
The end table would look like this:

Answer 1

You can access 'prior row' data by using something like:
select ID
, PaymentAmt
, PreviousPaymentAmt=LAG(PaymentAmt,1,0) OVER (ORDER BY ID)
from PStatement
The 2nd parameter in LAG is telling it to go only 1 record back; and the third parameter is telling it to return 0 (zero) if there are no prior rows.

Answer 2

Here is how I got it to work.
DECLARE #MyCursor CURSOR;
DECLARE #id INT, #Source NVARCHAR(255), #CBalance MONEY, #RBalance MONEY, #PaymentAmt MONEY, #AllocationPercent decimal(18,10), #PaymentNumber INT, #Amount MONEY;
BEGIN
SET #MyCursor = CURSOR FOR
SELECT ID FROM PStatement ORDER BY Source, PaymentNumber
OPEN #MyCursor
FETCH NEXT FROM #MyCursor
INTO #id
WHILE ##FETCH_STATUS = 0
BEGIN
SELECT #Source = Source, #PaymentNumber = PaymentNumber, #CBalance = CBalance, #PaymentAmt = PaymentAmt FROM PStatement WHERE ID = #id
IF(#PaymentNumber = 1)
BEGIN
UPDATE PStatement SET AllocationPercent = RBalance/ CAST(CBalance AS decimal(18, 10)) WHERE (ID = #id)
SET #AllocationPercent = (SELECT AllocationPercent FROM PStatement WHERE (ID = #id))
END
ELSE
BEGIN
SET #PaymentAmt = (SELECT SUM(PaymentAmt) FROM PStatement WHERE (PaymentNumber = #PaymentNumber))
SET #RBalance = ((#CBalance - #PaymentAmt) * #AllocationPercent) + #PaymentAmt
UPDATE PStatement SET RBalance = #RBalance WHERE (ID = #id)
UPDATE PStatement SET AllocationPercent = RBalance/ CAST(CBalance AS decimal(18, 10)) WHERE (ID = #id)
SET #AllocationPercent = (SELECT AllocationPercent FROM PStatement WHERE (ID = #id))
END
FETCH NEXT FROM #MyCursor
INTO #id
END;
CLOSE #MyCursor ;
DEALLOCATE #MyCursor;
END;