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

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