How to guarantee data consistency in two tables

Question

Hi
I have two tables debits and credits:
create table debits
client_id number;
amount_debit number
create table credits
client_id number;
amount_credit number
Now I need to guarantee that sum(amount_debit) - sum(amount_credit) > 0 for a specific client. Now I created a trigger on the credit table:
TRIGGER DEBIT_TRIGGER BEFORE INSERT ON "PPC"."DEBIT"
REFERENCING OLD AS OLD NEW AS NEW
FOR EACH ROW
declare
v_amount_debit number;
v_amount_credit number;
begin
select sum(amount_debit)
into v_amount_debit
from debit
where client_id = :new.client_id;
select sum(amount_credit)
into v_amount_credit
from credit
where client_id = :new.client_id;
if (v_amount_debit - v_amount_credit) < 0 then
raise_application_error(-20005, 'Error');
end if;
end;
But this won't work with multiple sessions because in two separates sessions an account can go below 0 before commit and the trigger won't detect the error.
In this case how can I guarantee data consistency?

Answer 1

You could SELECT...FOR UPDATE on the related row (in the "clients" table?) referenced by client_id, but...
1. Do you also need UPDATE and DELETE triggers on the "debits" table? How would you prevent an update of a debit (or a delete of a negative "debit) that causes the balance to drop below zero?
2. Do you also need (INSERT, UPDATE, and DELETE triggers) on the "credits" table? How would you prevent a delete or update of a credit (or an insert of a negative "credit") that causes the balance to drop below zero?
3. It might be easier to use a "refresh on commit" materialized view to enforce the constraint, something along the lines of the discussion at:
http://asktom.oracle.com/pls/ask/f?p=4950:8:::::F4950_P8_DISPLAYID:21389386132607
4. Another alternative might be to revoke insert, update, and delete privileges on the two tables and instead provide stored procedures (maybe named "debit" and "credit") that provide the necessary functionality and enforce the constraints (with locking).
Hope this helps.