My solution was incorrect. See the correct solution in the mentioned thread.

>Hi everybody,
>
>Here is an interesting SQL challenge (IMHO)
>
>http://forums.asp.net/p/1351246/2762267.aspx#2762267
>
>This is my solution.
>
>

>declare @Test table (ID int Identity, BatchID char(2), Quantity int)
>
>insert into @Test values ('B1', 100)
>insert into @Test values ('B1', 200)
>insert into @Test values ('B1', 300)
>insert into @Test values ('B1', 500)
>insert into @Test values ('B2', 500)
>
>declare @Amount int
>set @Amount = 500
>
>;with Cte_RunningTotal (ID, BatchID, Quantity, Total) as 
>(select *, (select SUM(Quantity) as Total from @Test T where T.BatchID = 'B1' AND Quantity <= T1.Quantity) from @Test T1 where T1.BatchID = 'B1')
>
>--select * from Cte_RunningTotal 
>
>update T set quantity = 
>
>case
>
>                  when Total <= @Amount then 0
>
>                  when Total > @Amount and Total - T.Quantity <= T.Quantity then (Total -@Amount)
>
>                 when Total > @Amount and Total - T.Quantity > T.Quantity then  T.Quantity end 
>                 from @Test T inner join Cte_RunningTotal on T.ID = Cte_RunningTotal.ID 
>
>select * from @Test 
>

>
>Do you think it is correct? What if I add records in different order?
>
>Thanks in advance.