SQL Question: How to do an "Easy" Substraction

[Tec-Nico]

Obviously is not as "easy" for me because I can't find a way to do it. But I hope it is for you.

Here is my problem:

I have 2 fields that I would like to substract. The problem is that the substraction is not very "natural".

I mean, this will be an example resulting set:

JoinId____|__QuantityA_____|__QuantityB
1________|______10_______|______40
1________|______32_______|______40
1________|______5________|______40
2________|______10_______|______15
2________|______10_______|______15

These two quantities are taken from different tables, lets say TableA and TableB... They are joined by the id I show (It is not a primary key)

What I would like to do is have something like this:

JoinId___|__QuantityA_____|__QuantityB___|___Result
1_______|______10_______|______40_____|_____0_(We substracted 10 from 40)
1_______|______32_______|______40_____|_____2_(We substract 32 from remaining 30)
1_______|______5________|______40_____|_____5_(We substract 5 from remaining 0)
2_______|______10_______|______15_____|_____0_(We substracted 10 from 15)
2_______|______10_______|______15_____|_____5_(We substract 10 from remaining 5)

It looks complicated but what I want to do is a substraction with every member of the same JoinId. The remaining would be substracted to the next one.

Example:

JoinId___|__QuantityA__|__QuantityB
1_______|______10____|______40____(40 - 10 = 30 so we will substract below)
1_______|______32____|______30____(30 - 32 = -2 so we won't substract below)
1_______|______5_____|______0_____

2_______|______10____|______15____(15 - 10 = 5 so we will substract below)
2_______|______10____|______5_____(5 - 10 = -5 so we won't substract below)


And so the results of this would be like this:

JoinId___|__QuantityA__|__QuantityB__|_Result
1_______|______10____|______40____|___0
1_______|______32____|______30____|___2
1_______|______5_____|______0_____|___5

2_______|______10____|______15____|___0
2_______|______10____|______5_____|___5


I hope I am explaining myself... Please let me know if I didn't. I think (I am not sure) it could be done using sub-queries and GROUP BY clauses... But I am not quite sure how to do it. Any help would be appreciated!

Thanks in advance!