Multiple entries alongwith one contra

Question

Hi,
I have a detail block in which i have fields like this and user can enter amount data in both fields (debit, credit) ...but i want to restrict my user that he/she should only enter data either MULTIPLE DEBITS and only one(1) credit entry like the following.
Acc_code....Name.................Debit.........Credit...................
1                Bank A             5000
2                Bank B             500
3                Bank C             450
4                Bank D             100          
10              Cash                                   6050I dont want my user to do the following.
Acc_code....Name.................Debit.........Credit...................
1                Bank A              5000
2                Bank B                500
3                Bank C                450
4                Bank D                100          
10              Cash                                   5000
10              Cash                                   1000
10              Cash                                      50OR this is also not allowed
Acc_code....Name.................Debit.........Credit...................
1                Bank A                                7000
2                Bank B                                1000
3                Bank C                                  500
4                Bank D                                  100          
10              Cash                   5000
10              Cash                   1000
10              Cash                   2000
10              Cash                     100In Short....following are the rules
1. AGAINST MULTIPLE DEBITS only one CREDIT entry is allowed
2. AGAINST MULTIPLE CREDITS only one DEBIT entry is allowed
3. ONE DEBIT AGAINST ONE CREDIT OR ONE CREDIT AGAINST ONE
DEBIT is allowed
4. NO REPEATATION OF ACC_CODES IN DETAIL BLOCK AS WELL
Now How to do this in forms .thanks in advance :)

Answer 1

Table Level Trigger
==============
Pre Insert
=======
Select count(*)
into V_count
from -----
and vouhcerid = ---
and credit_amt >0
if nvl(v_count,0) > 0 then
if :credit_amt <> 0 then
message/ alert show "only one credit entry pass"
raise form_trigger_failure;
end if;
end if;