oracle 触发器 插删改

 create or replace trigger TG_attend_417
before delete or insert or update on attend_417
for each row
begin
    if inserting then 
        update stud set sum_evaluation = sum_evaluation-2*(select count(*)from attend_417 where (attendance = 'la')-5*(select count(*)from attend_417 where attendance = 't')-1*(select count(*)from attend_417 where attendance = 'le')
        where sno=:new.sno;
    end if;
    if deleting then
        update stud set sum_evaluation = sum_evaluation-2*(select count(*)from attend_417 where (attendance = 'la')-5*(select count(*)from attend_417 where attendance = 't')-1*(select count(*)from attend_417 where attendance = 'le')
        where sno = :old.sno;
    end if;
    if updating then 
        update stud set sum_evaluation = sum_evaluation-2*(select count(*)from attend_417 where (attendance = 'la')-5*(select count(*)from attend_417 where attendance = 't')-1*(select count(*)from attend_417 where attendance = 'le')
        where sno = :old.sno;
    end if;
end;

这是代码，la表示迟到，扣2分，t表示旷课，扣5分，le表示请假，扣1分，attendance 表示出勤类别

请问要怎么修改才是正确的？

http://blog.csdn.net/indexman/article/details/8023740/
http://blog.csdn.net/justdo2008/article/details/4137779