如何为查询mysql添加临时值

Posted 2023-02-16

【中文标题】如何为查询mysql添加临时值

【英文标题】：how to add a temp values for query mysql

【发布时间】：2022-01-09 01:52:50

【问题描述】：

我的数据库中有以下查询，我想添加一个新列来处理学生的最终 Apperciation： 查询：

select student_name,q4.percentage
  from (select q2.student_id,mark *100/total as Percentage
          from (select class_id,sum(max_mark)as total
                  from course
                 group by(class_id)
               )q1 ,
               (select sum(mark) as mark,student_id
                  from grades
                 group by(student_id)
               ) q2
         where q2.student_id in (select student_id
                                   from student
                                  where student.section_id in(select section_id 
                                                                from section 
                                                               where class_id=q1.class_id)
                                )
         order by q2.student_id
       ) q4
inner join student on q4.student_id=student.student_id;

结果如下： enter image description here

STUDENT_NAME	PERCENTAGE
Massoud	50.41667
Ali-Shbeeb	84.16667
Mona	75.2941

现在我只需要在结果中添加一个新列，就像一个奖项一样 所以表格是这样的：

student_name     percetage award 
mahmoud-kabbani  79.166667 B
Kareem-Alshaeer  54.583    c

【问题讨论】：

在栏目奖项中决定提出哪个角色的标准是什么？

if percetage >80 is A if percetage >60 is B if percetage >40 is C else E

Why not upload images of code/errors when asking a question?，所以我在文本中添加了图片示例。

【参考方案1】：

您可以在下面的 case 语句中包含 1 列 -

select student_name,q4.percentage,
       CASE WHEN q4.percentage > 80 THEN 'A'
            WHEN q4.percentage > 60 THEN 'B'
            WHEN q4.percentage > 40 THEN 'C'
            ELSE 'E'
       END award
  from (select q2.student_id,mark *100/total as Percentage
          from (select class_id,sum(max_mark)as total
                  from course
                 group by(class_id)
               )q1 ,
               (select sum(mark) as mark,student_id
                  from grades
                 group by(student_id)
               ) q2
         where q2.student_id in (select student_id
                                   from student
                                  where student.section_id in(select section_id 
                                                                from section 
                                                               where class_id=q1.class_id)
                                )
         order by q2.student_id
       ) q4
inner join student on q4.student_id=student.student_id;