if objectproperty(object_id('B_UPD'),'IsTrigger')=1
drop trigger B_UPD
go
create trigger B_UPD
on B
for update
as
if update(POINT)
begin
    update A
        set GPA=case when t.CREDIT=0 then 0 else t.PC/t.CREDIT end
        from (
                select B.SNO,PC=sum(B.POINT*C.CREDIT), CREDIT=sum(C.CREDIT)
                    from C,B,inserted i
                    where C.CNO=B.CNO and B.SNO=i.SNO
                    group by B.SNO 
            )t
        where A.SNO=t.SNO
end
go