SQL——游标

游标是取用一组数据并能够一次与一个单独的数据进行交互的方法。关系数据库中的操作会对整个数据行集起作用。由SELECT语句返回的行集包括满足该语句的WHERE子句中条件的所有行。
一个完整的游标由5部分组成：
（1）声明游标
（2）打开游标
（3）从一个游标中查找信息
（4）关闭游标
（5）释放游标
1.使用游标打印OrderManagement库中各订单中的总金额，要求按总金额降序排。
声明游标

declare cur1 cursor
  for select 订单号, SUM(单价*数量)
	  from order_detail
	  group by(订单号)
	  order by  SUM(数量*单价)

打开游标

open cur1
  declare @ono char(6)
  declare @mon int
  fetch next from cur1 into @ono, @mon   
  print '订单号  总金额'
  print '--------------'
  while @@FETCH_STATUS = 0
	begin
	 print @ono + '  ' + cast(@mon as char(5))
	 fetch next from cur1 into @ono, @mon
	end

关闭游标

close cur1

释放游标

deallocate cur1

2.使用游标提取学生课程库中3-105课的前三名学生的信息和后三名学生的信息，包括学号、姓名、课程名和成绩。（不考虑成绩相同的情况）

declare cur2 scroll cursor
	for select student.sno, sname, cname, degree
        from student, course, score
        where student.sno = score.sno and score.cno = course.cno and course.cno = '3-105'
        order by degree
     open cur2
     declare @ssno char(5)
     declare @ssname char(10)
     declare @ccname  char(16)
     declare @ddgree float
--     fetch next from cur2		--提取下一行
--     fetch last from cur2	         	--提取最后一行
--     fetch prior from cur2		--提取当前行的前一行
	 print '学号   姓名      课程名      成绩'
	 print '---------------------------------' 
     fetch absolute 1 from cur2 into @ssno, @ssname, @ccname, @ddgree --提取第一行
     print @ssno + @ssname + @ccname + cast(@ddgree as char(5))
     fetch absolute 2 from cur2 into @ssno, @ssname, @ccname, @ddgree --提取第二行
     print @ssno + @ssname + @ccname + cast(@ddgree as char(5))
     fetch absolute 3 from cur2 into @ssno, @ssname, @ccname, @ddgree --提取第三行
     print @ssno + @ssname + @ccname + cast(@ddgree as char(5))
     fetch last from cur2 into @ssno, @ssname, @ccname, @ddgree         --提取最后一行
     print @ssno + @ssname + @ccname + cast(@ddgree as char(5))
     fetch prior from cur2 into @ssno, @ssname, @ccname, @ddgree     --提取当前行的前一行
     print @ssno + @ssname + @ccname + cast(@ddgree as char(5))
     fetch prior from cur2 into @ssno, @ssname, @ccname, @ddgree 
     print @ssno + @ssname + @ccname + cast(@ddgree as char(5))
     close cur2
     deallocate cur2

3.使用游标修改OrderManagement库的order_detail表中订单号为***的单价最高的器件的单价和单价最低的器件的单价。

  declare cur3 scroll cursor
    for select *
        from order_detail
        where 订单号 = 'OR-01C'
        order by 单价
    for update
    open cur3
    fetch first from cur3
    update order_detail
    set 单价 = 100
    where current of cur3
    fetch last from cur3
    update order_detail
    set 单价 = 200
    where current of cur3
    close cur3
    deallocate cur3

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