SQL Oracle触发器错误

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.