Question

来自以下的Mysql查询

select (
   case
   when grade < 8 then null 
   else name
   end
   ) as name,
   grade, marks 
from students
join Grades
ON Min_mark = (SELECT MAX(Min_mark) from grades where min_mark <= students.marks)
order by grade desc,name asc

我想根据等级的值对订单进行排序，即

生成包含三列的报告：名称，成绩和标记。报告必须按等级降序排列 - 即首先输入更高的等级。如果分配了相同年级（1-10）的学生不止一个，请按字母顺序按名称命名这些学生。最后，如果等级低于8，请使用＆＃34; NULL＆＃34;作为他们的名字，并按他们在升序中的标记列出

select (
   case
   when grade < 8 then null 
   else name
   end
   ) as name,
   grade, marks 
from students
join Grades
ON Min_mark = (SELECT MAX(Min_mark) from grades where min_mark <= students.marks)
 case
    when grade <8 then 
        order by grade desc, name asc
    else 
        order by marks asc
 end

但由于语法错误，第二个查询无法执行。有人可以建议我如何获得所需的结果。在此先感谢

Answer 1

我的猜测是这样的：

ORDER BY grade > 10 DESC /* orders by grade>10 records first */
, CASE WHEN grade > 10 THEN grade ELSE null END DESC /* all grade<=10 records will "sort" equally among themselves */
, CASE WHEN grade > 10 THEN name ELSE marks END ASC /* all grade<=10 records will "sort" by `name`, others will sort my `marks`  */

Answer 2

不可能通过两个不同的order by子句对查询进行排序。

您可以在语句中添加一个或多个其他行，并按此行排序。要做到这一点，你必须决定如何排序。如果等级低于10，则所有等级之前的等级> 10？

这样的事情可行：

select (
   case
   when grade < 8 then null 
   else name
   end
   ) as name,
   grade, marks,
   (
   case
   when grade > 10 then grade
   else null
   end
   ) as sortorder1,
   (
   case
   when grade > 10 then name
   else marks
   end
   ) as sortorder2 
from students
join Grades
ON Min_mark = (SELECT MAX(Min_mark) from grades where min_mark <= students.marks)
order by sortorder1 desc, sortorder2 asc

编辑低于8的成绩也应按成绩排序：

我认为这应该有效：

select (
   case
   when grade < 8 then null 
   else name
   end
   ) as displayname,
   grade, marks 
from students
join Grades
ON Min_mark = (SELECT MAX(Min_mark) from grades where min_mark <= students.marks)
order by grade desc, displayname asc, marks asc

我将＆＃39;替换为名称＆＃39;由＆＃39;作为显示名称＆＃39;为了更清楚，排序应该通过displayname来完成，我不确定数据库是否需要这样做，但对于阅读查询的人来说更清楚。

Answer 3

我不知道你的专栏名称，但对我而言，这应该是这么简单：

select IF(grade < 8, null, grade) as processed_grade, name 
from students
join Grades
ON students.id = Grades.id_student
order by processed_grade desc,name asc

如果低于8，您只需将成绩更改为null，然后按此新创建的列和名称对记录进行排序。

如何在mysql中按升序和降序修改一列

3 个答案: