Simple Query to Calculate FIFO

I assume that you have table below :
tbl_purchasing
-------------------------------------------------------------------------------------
purchase_number - item_id - date_purchase - unit_cost - qty  - last_qty
-------------------------------------------------------------------------------------
001 - A2 -11/11/2012 - 1000 - 120 - 50
002 - A2 -11/11/2012 - 1100 - 100 - 100
003 - A2 -13/11/2012 - 1350 - 10 - 10
-------------------------------------------------------------------------------------
Note : last_qty is available qty to sale. In this case, it mean 70 was sold.

Sample case :
at 14/11/2012 we need to input sales invoice with item A2 and qty = 152.
How to calculate price using FIFO method?
Aha! as easy as you think.. :p lol
A simple query will solve that :

set @param:=40;
set @counter:=40;

select purchasing.purchase_number, purchasing.sisa,purchasing.unit_cost,
case when @param <= purchasing.sisa then @param else purchasing.sisa end * unit_cost harga,

@param := @param - purchasing.sisa as sampah_masyarakat

from
   (
        select doc_id,sum(last_qty) sisa,unit_cost
        from tbl_purchasing
        where item_id = 'A2'
        group by unit_cost
    order by date_purchase,purchase_number
     ) purchasing
having (harga > 0)

just change 40,A2 into variable and ..
Done!

Very simple, even dont need to create func,proc,trigger, whatever..
Juat a simple single query!
Created by me with some reference from stackoverflow.com

Enjoy..
Cheers!


Note : tested in mysql not sure run in another..