forward rolling sum with different stopping points by row - tsql

Question

First, some sample data so the business problem can be explained -
select
ItemID = 276,
Quantity,
Bucket,
DaysInMonth = day(eomonth(Bucket)),
DailyQuantity = cast(Quantity * 1.0 / day(eomonth(Bucket)) as decimal(4, 0)),
DaysFactor
into #data
from
(
values
('1/1/2021', 95, 5500),
('2/1/2021', 75, 6000),
('3/1/2021', 80, 5000),
('4/1/2021', 82, 5300),
('5/1/2021', 90, 5200),
('6/1/2021', 80, 6500),
('7/1/2021', 85, 6100),
('8/1/2021', 90, 5100),
('9/1/2021', null, 5800),
('10/1/2021', null, 5900)
) d (Bucket, DaysFactor, Quantity);
select * from #data;
Now, the business problem -
The first row has a DaysFactor of 95.
The forward rolling sum for this row is calculated as
(31 x 177) + (28 x 214) + (31 x 161) + (5 x 177) = 17,355
That is...
the daily quantity for all 31 days of the 1/1/2021 bucket plus
the daily quantity for all 28 days of the 2/1/2021 bucket plus
the daily quantity for all 31 days of the 3/1/2021 bucket plus
the daily quantity for 5 days of the 4/1/2021 bucket.
This results in 95 days of forward looking quantity.
95 days = 31 + 28 + 31 + 5
For the second row, with a DaysFactor of 75, it would start with daily quantity for the 28 days in the 2/1/2021 bucket and go out until a total of 75 days' worth of quantity were summed, like so:
(28 x 214) + (31 x 161) + (16 x 177) = 13,815
75 days = 28 + 31 + 16
One approach to this is building a calendar of daily demand and then summing quantity over the specified days. However, I'm stuck on how to do the summing. Here is the code that builds the calendar with daily quantities:
with
dates as
(
select
FirstDay = min(cast(Bucket as date)),
LastDay = eomonth(max(cast(Bucket as date)))
from #data
),
tally as (
select top (select datediff(d, FirstDay, LastDay) + 1 from dates) --restrict to number of rows equal to number of days between first and last days
n = row_number() over(order by (select null)) - 1
from sys.messages
),
calendar as (
select
Bucket = dateadd(d, t.n, d.FirstDay)
from tally t
cross join dates d
)
select
c.Bucket,
d.DailyQuantity
from #data d
inner join calendar c
on year(d.Bucket) = year(c.Bucket)
and month(d.Bucket) = month(c.Bucket);
Here's a screenshot of a subset of rows from this query:
I was hoping to use T-SQL's LEAD() to do this but don't see a way to put the DaysFactor into the ROWS clause within OVER(). Is there a way to do that? If not, is there a set based approach to calculating the rolling forward sum?
Expected result set:

Answer 1

Figured it out using an approach different than LEAD(). This column was added to #data:
BucketEnd = cast(dateadd(d, DaysFactor - 1, Bucket) as date)
Then code that builds the calendar with daily quantities shown in original question was put into a temp table called #calendar.
Then this query performs the calculations:
select
d.ItemID,
d.Bucket,
RollingForwardQuantitySum = sum(iif(c.Bucket between d.Bucket and d.BucketEnd, c.DailyQuantity, null))
from #data d
cross join #calendar c
group by
d.ItemID,
d.Bucket
order by
d.ItemID,
cast(d.Bucket as date);
The output from this query matches the expected result set screen shot in the original post.