add and subtract column in query

[luciffer]

I don't have the db on this computer but here is what the data looks like.

draws              
-----             
1  10/07/2004           
2  17/07/2004     
3  24/07/2004
4  31/07/2004

ticket_sales
-------------
ID ticket_id draw_id participant_id amount_paid
-- --------- ------- -------------- -----------
1  100       1       5000           5          
2  100       2       5000           0
3  100       3       5000           0
4  100       4       5000           2
5  200       1       6000           0
6  200       2       6000           0
7  200       3       6000           0
8  200       4       6000           5

getting the credits for ticket_id = 100 and draw_id = 4 would yield 3
getting the credits for ticket_id = 200 and draw_id = 4 would yield 4

DaPi: I hope the second case helps show why its not sum(payments) - count(tickets). As sum(payments) would be 5 and count(tickets) would be 4 which is 1 and incorrect.