create or replace trigger stu_trig
after delete or insert or UPDATE on tb_student
declare
    age number;
    cnt number;
begin
    if inserting then
        select min(TRUNC(months_between(sysdate, birthday)/12)) into age from tb_student;
        dbms_output.put_line(age);
    elsif deleting then
        select count(*) into cnt from tb_student;
        dbms_output.put_line(cnt);
    elsif updating then
         select avg(TRUNC(months_between(sysdate, birthday)/12)) into age from tb_student;
        dbms_output.put_line(age);
    end if;
end stu_trig;