create or replace procedure
insert_duration (d_code in number,f_date in date,t_date in date) as
n_code duration.duration_code%type;
begin
   select nvl(max(duration_code),0) into n_code from duration;
   n_code := n_code + 1;
   if n_code = d_code then
      insert into duration values(d_code , f_date , t_date);
      commit;
   else
      raise_application_error(-20101,'Duration code out of sequence');
   end if;
end;
/