1:    写一个存储过程实现求学生的平均总成绩（必须用游标实现)

if exists(select * from sysobjects where name='st_cursor' and type='p')
    drop procedure st_cursor
go
create procedure st_cursor @st_cursor cursor varying output
as
begin
        declare @aver int
    set @st_cursor=cursor forward_only static for
       select @aver=(select avg(成绩) from xs_kc
     group by 学号)
       return @aver
end
    open @st_cursor
go
        declare @mycursor cursor
    execute st_cursor @st_cursor=@mycursor output
    while (@@fetch_status=0) /*上一个获取成功*/
        begin
            fetch next from @mycursor
        end
    close @mycursor
       deallocate @mycursor


出错信息:
在 游标声明 中不允许使用 变量赋值。

但是这个书上的例子为什么编译通过了呢?

if exists(select * from sysobjects where name='st_cursor' and type='p')
    drop procedure st_cursor
go
create procedure st_cursor @st_cursor cursor varying output
as
    set @st_cursor=cursor forward_only static for
        select * from xs
    open @st_cursor

go
        declare @mycursor cursor
    execute st_cursor @st_cursor=@mycursor output
    while (@@fetch_status=0)/*上一个获取成功*/
        begin
            fetch next from @mycursor
        end
    close @mycursor
       deallocate @mycursor