I have created a trigger on a table like
*****************************************

create or replace trigger update_sal
before update on emp
for each row
when (new.sal < old.sal)
begin
raise_application_error (-20507 , 'Sorry ........');
end;

This trigger works Fine

But problem with this tigger
****************************

create or replace trigger insert_record
before insert on table
for each row
when (new.id = old.id)
begin
raise_application_error (-20007 , 'Sorry ......');
end;

I wanna create trigger.. Any Help
check the column value for duplication.
I know about the unique constraint and primary key.