Sql oracle trigger error - sql

Question

Hi I was wondering if someone can help me adjust my trigger. I have only been working with triggers for only couple of weeks and quite new to it. My trigger is suppose to add $75 per day a boat is return late to the customer_balance account. If the boat is return early then it adds $20 per day the boat was return early to the customer_Balance account. I keep getting errors trying to get this trigger to work right so I was wondering can help or explain to me what I am doing wrong. Thank you.
create or replace trigger trig_penalty
before update of customer_balance
on customer for each row
declare
i number;
s varchar(20);
begin
select customer_balance into s
from customer
where customer_ID =:new.customer_id;
i := :new.charter_returndate - :old.charter_duedate;
if i>=0 then
dbms_output.put_line('Late return fee added');
update customer
set customer_balance=customer_balance+75*i
where customer_id=s;
else
dbms_output.put_line('Early return refund added');
update customer
set customer_balance=customer_balance+20*i
where customer_id=s;
end if;
end;
/

Answer 1

Well you have to understand that a trigger is an artifact that work for (in your case) each row that are being modified. So you are doing unecessary work on it.
Try this:
create or replace trigger trig_penalty
before update of customer_balance
on customer for each row
declare
i number;
s varchar(20);
begin
--You dont need to select the id is it is already on :new.customer_id
-- your trigger is for UPDATE
---select customer_balance into s
--- from customer
--- where customer_ID := :new.customer_id;
i := :new.charter_returndate - :old.charter_duedate;
if i>=0 then
--Do not use dbms_output in an internal object.
--dbms_output.put_line('Late return fee added');
--You don't need to run an update for an update trigger you
-- just need to set the new value on :NEW.FIELD
--update customer
-- set customer_balance=customer_balance+75*i
-- where customer_id=s;
--As you select this value I'm using the OLD which means the value that
--Was already on the table
:NEW.customer_balance := :OLD.customer_balance+75*i;
else
--dbms_output.put_line('Early return refund added');
--update customer
-- set customer_balance=customer_balance+20*i
-- where customer_id=s;
:NEW.customer_balance := :OLD.customer_balance+20*i;
end if;
end;
/
That should do it. Remeber for an BEFORE trigger the value setted on :NEW will end up on the table.