SQL SERVER 触发器问题(100分)

我在GRADE 表中定义如下的触发器：
（其作用是当GRADE表中的数据发生变化的时候，重新计算各科总分，然后排序）
CREATE TRIGGER G_order ON grade
FOR UPDATE
AS
begin
update grade set
total =s1+s2+s3+s4+s5+s6+s7+s8+s9+s10+s11
--定义游标
declare G_order_cur scroll cursor for
Select Grade.Total,Grade.GradeOrder,Grade.Studentno,student.stuno,student.classno
from grade,student,classinfor,inserted
where grade.studentno=student.stuno and
student.classno=classinfor.classno and
classinfor.grade in
(select classinfor.grade from classinfor,student,Grade
where inserted.studentno=student.stuno and student.classno=classinfor.classno )
and Grade.term=inserted.term and Grade.test=inserted.test
order by Grade.total desc
--打开游标
open G_order_Cur
declare
@P_total int,
@N_order int,
@N_total int,
@n int,
@P_order int
--排序操作
select n=1
update Grade set Gradeorder=@n where current of G_order_Cur
fetch first from G_order_cur into @P_Total,@p_order
loop:
begin
select @n=@n+1
if @N_total=@p_total
update Grade set Gradeorder=@P_order where current of G_order_Cur
else
update Grade set Gradeorder=@n where current of G_order_Cur
fetch next from G_order_cur into @N_Total,@N_order
if @@fetch_Status=0 goto loop
end
--关闭游标
close G_order_cur
--删除游标
deallocate G_order_cur
end
当我在SQl Server Query Anasyse中执行以下语句
update grade set s8=9
其显示的结果为
(60 row(s) affected)
n
-----------
1
(1 row(s) affected)
Server: Msg 16929, Level 16, State 1, Procedure G_order, Line 25
The cursor is READ ONLY.
The statement has been terminated.
请教各位大虾是何原因？
（另外，还想请教怎样才能使当有多行数据更新时，
使触发器只执行一次（当然是最后一次执行）以提高效率）

我也想听

《数据库原理》一书中的标准答案：
如果游标定义中的select用了order by ,union,或相当于定义了
不可更新的视图，就不能用
update s set .... where current of sc
or
delete from ... where current of sc

接受答案了.