create table app_sri
(a_id integer primary key,
p_first_name varchar2(30),
p_last_name varchar2(20),
p_age number(3)
);

create or replace trigger pid_trg
  before insert on app_sri
  for each row
begin
select pid_seq.nextval into :new.a_id from dual;
end;

Begin

insert into app_sri(null,'robo','Big',100);
commit

End;