Trigger Sql Oracle

Question

create or replace trigger t1 
  before update of price on book 
declare 
  vdif number; 
begin 
  vdif:=:new.price-:old.price; 
  dbms_output.put_line('Price Diff is '||vdif); 
end;

I'm getting this error:

NEW or OLD references not allowed in table level triggers

Answer 1

As the error message says, you can't use :new or :old in a table-level trigger. You need a row-level trigger. So you'd need to add FOR EACH ROW to your declaration

create or replace trigger t1 
  before update of price on book 
  for each row
declare 
  vdif number; 
begin 
  vdif:=:new.price-:old.price; 
  dbms_output.put_line('Price Diff is '||vdif); 
end;

Of course, in reality, you'd never write a trigger that just wrote to the dbms_output buffer (nor would you write production code that depended on anyone seeing anything written to dbms_output ). But I assume that you are a student and you're just doing this as part of a homework assignment.

Answer 2

Trigger you wrote is a table level trigger, and table level triggers fire once for each operation on the table. So, for example, if you had a query that updates multiple rows, the trigger would be called just once so :new and :old doesn't know what row to affect.

What you actually want is to add a FOR EACH ROW clause in the trigger definition (under "before update..."), which will make your trigger a row level trigger and it will fire for each row you are updating.