T-SQL - Filling in the gaps in running balance - sql-server

Question

I am working on a Data Warehouse project and the client provides daily sales data. On-hand quantities are provided in most lines but are often missing. I need help on how to fill those missing values based on prior OH and sales information.
Here's a sample data:
Line# Store Item OnHand SalesUnits DateKey
-----------------------------------------------
1 001 A 100 20 1
2 001 A 80 10 2
3 001 A null 30 3 --[OH updated with 70 (80-10)]
4 001 A null 5 4 --[OH updated with 40 (70-30)]
5 001 A 150 10 5 --[OH untouched]
6 001 B null 4 1 --[OH untouched - new item]
7 001 B 80 12 2
8 001 B null 10 3 --[OH updated with 68 (80-12]
Lines 1 and 2 are not to be updated because OnHand quantities exist.
Lines 3 and 4 are to be updated based on their preceding rows.
Line 5 is to be left untouched because OnHand is provided.
Line 6 is to be left untouched because it is the first row for Item B
Is there a way I can do this in a set operation? I know I can do it easily using a fast_forward cursor but it will take a long time (15M+ rows).
Thanks for your help!

Answer 1

Test data:
declare #t table(
Line# int, Store char(3), Item char, OnHand int, SalesUnits int, DateKey int
)
insert #t values
(1, '001', 'A', 100, 20, 1),
(2, '001', 'A', 80 , 10, 2),
(3, '001', 'A', null, 30, 3),
(4, '001', 'A', null, 5, 4),
(5, '001', 'A', 150, 10, 5),
(6, '001', 'B', null, 4, 1),
(7, '001', 'B', null, 4, 2),
(8, '001', 'B', 80, 12, 3),
(9, '001', 'B', null, 10, 4)
Script to populate not using cursor:
;with a as
(
select Line#, Store, Item, OnHand, SalesUnits, DateKey, 1 correctdata from #t where DateKey = 1
union all
select t.Line#, t.Store, t.Item, coalesce(t.OnHand, a.onhand - a.salesunits), t.SalesUnits, t.DateKey, t.OnHand from #t t
join a on a.DateKey = t.datekey - 1 and a.item = t.item and a.store = t.store
)
update t
set OnHand = a.onhand
from #t t join a on a.line# = t.line#
where a.correctdata is null
Script to populate using cursor:
declare #datekey int, #store int, #item char, #Onhand int,
#calculatedonhand int, #salesunits int, #laststore int, #lastitem char
DECLARE sales_cursor
CURSOR FOR
SELECT datekey+1, store, item, OnHand -SalesUnits, salesunits
FROM #t sales
order by store, item, datekey
OPEN sales_cursor;
FETCH NEXT FROM sales_cursor
INTO #datekey, #store, #item, #Onhand, #salesunits
WHILE ##FETCH_STATUS = 0
BEGIN
SELECT #calculatedonhand = case when #laststore = #store and #lastitem = #item
then coalesce(#onhand, #calculatedonhand - #salesunits) else null end
,#laststore = #store, #lastitem = #item
UPDATE s
SET onhand=#calculatedonhand
FROM #t s
WHERE datekey = #datekey and #store = store and #item = item
and onhand is null and #calculatedonhand is not null
FETCH NEXT FROM sales_cursor
INTO #datekey, #store, #item, #Onhand, #salesunits
END
CLOSE sales_cursor;
DEALLOCATE sales_cursor;
I recommand you use the cursor version, I doubt you can get a decent performance using the recursive query. I know people in here hate cursors, but when your table has that size, it can be the only solution.