SQL amount consumed query? - sql

Question

How do you do a query to calculate how much of an item has been consumed (used up)?
We can find the qty of each item that we purchased in a purchases table with columns Id, ProductId, Qty (decimal), Date
Id, ProductId, Qty, Date
1, 1, 10, 1/1/11
2, 1, 5, 2/2/11
3, 1, 8, 3/3/11
And how do you then add a count of how many of each row in the purchase table have been consumed - assuming strict FIFO? So in the above example if we know that 14 have been consumed the output would be:
Id, ProductId, Qty, Date, Consumed
1, 1, 10, 1/1/11, 10
2, 1, 5, 2/2/11, 4
3, 1, 8, 3/3/11, 0
Hopefully that explains what I mean by an amount consumed query - we know 14 were consumed and that the first purchase was for 10, so all 10 have been consumed. The next purchase was for 5 so we know that 4 of those have been consumed.
Theres two places I can get the consumed data from - the ConsumedItems table: columns Id, ProductId, QtyUsed, Date), or from the ConsumedSummaryView with columns ProductId, QtyUsed (this is the sum of ConsumedItems.QtyUsed)

Answer 1

Sample table and view
create table purchases (Id int, ProductId int, Qty int, Date datetime)
insert purchases select 1, 1, 10, '1/1/11'
insert purchases select 2, 1, 5, '2/2/11'
insert purchases select 3, 1, 8, '3/3/11'
create view ConsumedSummaryView as select ProductID = 1, QtyUsed = 14
The query
;with p as (
select *, rn=ROW_NUMBER() over (partition by productid order by date, id)
from purchases)
, tmp(Id, ProductId, Qty, Date, rn, ToGo, Consumed) as (
select p.Id, p.ProductId, p.Qty, p.Date, cast(1 as bigint),
CAST(ISNULL(v.qtyused,0) - p.Qty as decimal(20,10)),
cast(case
when v.qtyused >= p.Qty Then p.Qty
when v.qtyused > 0 then v.qtyused
else 0 end as decimal(20,10))
from p
left join ConsumedSummaryView v on p.ProductId = v.productId
where rn=1
union all
select p.Id, p.ProductId, p.Qty, p.Date, cast(p.rn as bigint),
cast(ISNULL(tmp.toGo,0) - p.Qty as decimal(20,10)),
cast(case
when tmp.toGo >= p.Qty Then p.Qty
when tmp.toGo > 0 then tmp.toGo
else 0 end as decimal(20,10))
from tmp
--inner join p on p.rn=tmp.rn+1
inner join p on p.rn=tmp.rn+1 and p.productid = tmp.ProductId
)
select Id, ProductId, Qty, Date, Consumed
from tmp
order by rn
Output
Id ProductId Qty Date Consumed
----------- ----------- ----------- ----------------------- -----------
1 1 10 2011-01-01 00:00:00.000 10
2 1 5 2011-02-02 00:00:00.000 4
3 1 8 2011-03-03 00:00:00.000 0

Answer 2

A little different approach than Richard's, but I'm not sure which will perform better:
SELECT
Purchases.Id,
Purchases.ProductId,
Purchases.Qty,
Purchases.Date,
CASE
WHEN COALESCE (PreviousPurchases.PreviousUsed, 0) + Qty < ConsumedSummaryView.QtyUsed THEN Qty
ELSE
CASE
WHEN ConsumedSummaryView.QtyUsed - COALESCE (PreviousPurchases.PreviousUsed, 0) < 0 THEN 0
ELSE ConsumedSummaryView.QtyUsed - COALESCE (PreviousPurchases.PreviousUsed, 0)
END
END AS Used
FROM
Purchases
INNER JOIN ConsumedSummaryView ON Purchases.ProductId = ConsumedSummaryView.ProductId
LEFT OUTER JOIN (
SELECT
SUM(Purchases_2.Qty) AS PreviousUsed,
Purchases_1.Id
FROM
Purchases AS Purchases_2
INNER JOIN Purchases AS Purchases_1 ON Purchases_2.Id < Purchases_1.Id
AND Purchases_2.ProductId = Purchases_1.ProductId
GROUP BY
Purchases_1.Id
) AS PreviousPurchases ON Purchases.Id = PreviousPurchases.Id