Running Totals Skipping Over Certain Values - sql-server

Question

I need to calculate a column that has a value column subtracted from a Total column but can skip rows until it can no longer find a smaller value. The sequence relates to dates so the order must be preserved. The value (Need) cannot be larger than the total as those are deleted prior.
This is for SQL Server 2016. My initial thought process was to use window functions and a running total, but I cannot figure out how to skip the 400 and continue to the 2 rows below. I included my attempts in the CASE statement as TransferQty and the running total as ReferenceCol.
Code to reproduce:
DECLARE #i TABLE
(
sequence INT IDENTITY(1,1)
,Total INT
,Need INT
)
INSERT INTO #i
VALUES (500,100)
,(500,200)
,(500,50)
,(500,400)
,(500,50)
,(500,50)
SELECT
sequence
,Total
,Need
,CASE
WHEN Total - SUM(Need) OVER (ORDER BY sequence) > 0
THEN Need
ELSE 0
END AS TransferQty
,Total - SUM(Need) OVER (ORDER BY sequence) as ReferenceCol
FROM #i
Current Results
+----------+-------+------+-------------+--------------+
| Sequence | Total | Need | TransferQty | ReferenceCol |
+----------+-------+------+-------------+--------------+
| 1 | 500 | 100 | 100 | 400 |
| 2 | 500 | 200 | 200 | 200 |
| 3 | 500 | 50 | 50 | 150 |
| 4 | 500 | 400 | 0 | -250 |
| 5 | 500 | 50 | 0 | -300 |
| 6 | 500 | 50 | 0 | -350 |
+----------+-------+------+-------------+--------------+
Desired Results
+----------+-------+------+-------------+--------------+
| Sequence | Total | Need | TransferQty | ReferenceCol |
+----------+-------+------+-------------+--------------+
| 1 | 500 | 100 | 100 | 400 |
| 2 | 500 | 200 | 200 | 200 |
| 3 | 500 | 50 | 50 | 150 |
| 4 | 500 | 400 | 0 | 150 | --skip calc
| 5 | 500 | 50 | 50 | 100 |
| 6 | 500 | 50 | 50 | 50 |
+----------+-------+------+-------------+--------------+

Answer 1

You should be able to use this code if you have a single skip, but when you have multi skips then you have to loop through and perform the delete of record based on the existence of rolling value exceeding the total.
DECLARE #i TABLE
(
sequence INT IDENTITY(1,1)
,Total INT
,Need INT
)
INSERT INTO #i
VALUES
(500,100 )
,(500,200 )
,(500,50 )
,(500,400 )
,(500,50 )
,(500,50 )
select sequence,Total,Need
into #temp_original
from #i
select
b.sequence,b.Total, SUM( a.need) rollingvalue ,
case when SUM( a.need) > b.Total
then 0
when SUM( a.need) = b.Total then SUM( a.need)
else b.Total - SUM( a.need) end how_much_needed
into #temp
from #i a
join #i b
on a.sequence < b.sequence + 1
group by b.sequence,b.Total
delete from a
from #i a
join (
select min(sequence) min_sequence
from #temp
where how_much_needed = 0
) minseq
on minseq.min_sequence = a.sequence
select
b.sequence,b.Total, SUM( a.need) rollingvalue ,
case when SUM( a.need) > b.Total
then 0
when SUM( a.need) = b.Total then SUM( a.need)
else b.Total - SUM( a.need) end how_much_needed
into #temp2
from #i a
join #i b
on a.sequence < b.sequence + 1
group by b.sequence,b.Total
select a.sequence,a.Total,a.Need, case when isnull (b.rollingvalue , 0) = 0 then 0 else case when b.rollingvalue > a.Total then 0 else a.Need end end as TransferQty , ISNULL( case when b.how_much_needed = b.Total then a.Need else b.how_much_needed end, case when ( select how_much_needed from #temp2 where sequence = a.sequence -1) = a.Total then 0 else (select how_much_needed from #temp where sequence = a.sequence -1) end ) ReferenceCol
from #temp_original a
LEFT join #temp2 b
on a.sequence = b.sequence
join #temp c
on c.sequence = a.sequence
drop table #temp
drop table #temp2
drop table

Answer 2

Here is the solution I went with which is based off of the "Quirky Update" from the original comment.
DROP TABLE IF EXISTS #i
GO
CREATE TABLE #i
(
sequence INT IDENTITY(1,1) PRIMARY KEY CLUSTERED
,Total INT
,Need INT
,RunningTransfer INT NULL
)
INSERT INTO #i
VALUES
(500,100,NULL)
,(500,200,NULL)
,(500,50,NULL)
,(500,400,NULL)
,(500,50,NULL)
,(500,50,NULL)
,(500,100,NULL)
,(500,49,NULL)
,(500,50,NULL)
DECLARE #TransferRunningTotal INT
UPDATE #i
SET #TransferRunningTotal = RunningTransfer = CASE
--this skips values larger than running total
WHEN #TransferRunningTotal < Need THEN #TransferRunningTotal
--this creates the running total
WHEN #TransferRunningTotal > Need THEN #TransferRunningTotal - Need
--creates the initial value
ELSE Total - Need
END
FROM #i WITH (TABLOCKX)
OPTION (MAXDOP 1)
SELECT sequence
,Total
,Need
,CASE
WHEN need <= RunningTransfer THEN Need
ELSE 0
END AS TsfQty
,RunningTransfer
FROM #i