Updating field using trigger

Question

I have a very basic table with four fields, DateReturned DueDate, daysoverdue and days. I am trying to use the following code to calculate the difference between the two dates and enter the result into days overdue. After this calculation I want to transfer only the positive values into the days field.
Calculation 1 - based on When-Button-Pressed
update test1 set daysoverdue = RETURNDATE - DUEDATE;
I can get this to work using the formula option in Property Palette - is there any way I can use an if statement. I only really want to calculate days overdue if the return date is after due date so that I have positive values only - this would avoid the need for calculation two.
Calculation two - - based on When-Button-Pressed
update test1 set days = RETURNDATE - DUEDATE
where daysoverdue >0;
When I compile and press the button - my application crashes. I can run the code via SQLPlus and it works.
What I would really like to do is the run the code automatically then the records are called up into the form. Any ideas and suggestions welcome. My PC that I am using is old and is a W2K machine with 256mb Ram and Approx 600Mhz processor.
This is my first application I am developing so if I have left out information - sorry.

Answer 1

my small advice is , pls do not write any
insert , update or delete in forms
if you want to do that pls call a database procedure
for ex
pls call a foll procedure in when-button-pressed
trigger
dp_update_overdue;
and write the foll script in your oracle database
create or replace procedure dp_update_overdue
is
begin
update test1
set daysoverdue = returndate- duedate
where returndate > duedate;
commit;
end;
bye
pasupathy