MySql 中的 WHEN 条件生成多列而不是一列？

Question

我有两个表格（学生和分数），如下图所示：

学生 TABLE:

标记 TABLE:

查询：

select s.*, m.math+m.English+m.science+m.hindi+m.s_science as Total,
    CASE WHEN m.math < 30 THEN 'Fail in Math' else '' END,
    CASE WHEN m.English < 30 THEN 'Fail in English' else '' END,
    CASE WHEN m.science < 30 THEN 'Fail in science' else '' END,
    CASE WHEN m.Hindi < 30 THEN 'Fail in Hindi' else '' END,
    CASE WHEN m.s_science < 30 THEN 'Fail in s_science' else '' END
AS Remark,

CASE WHEN (m.math+m.English+m.science+m.hindi+m.s_science) < 150 THEN 'Fail' else 'Pass' END AS Result
from marks as m
join student as s
on m.st_id = s.st_id

结果：

预期输出：列备注将创建一次，最后的备注将在单列中。

Answer 1

您的查询中存在语法问题。试试这个。

如果你只想显示一个备注。

select s.*, m.math+m.English+m.science+m.hindi+m.s_science as Total,
    CASE WHEN m.math < 30 THEN 'Fail in Math' 
    WHEN m.English < 30 THEN 'Fail in English' 
    WHEN m.science < 30 THEN 'Fail in science'
    WHEN m.Hindi < 30 THEN 'Fail in Hindi'
    WHEN m.s_science < 30 THEN 'Fail in s_science' else '' END
AS Remark,

CASE WHEN (m.math+m.English+m.science+m.hindi+m.s_science) < 150 THEN 'Fail' else 'Pass' END AS Result
from marks as m
join student as s
on m.st_id = s.st_id

否则，如果您想合并列中的所有备注，请使用 concat。由于有多个主题标记。

select s.*, m.math+m.English+m.science+m.hindi+m.s_science as Total,
   Concat( CASE WHEN m.math < 30 THEN 'Fail in Math' else '' END,
    CASE WHEN m.English < 30 THEN 'Fail in English' else '' END,
    CASE WHEN m.science < 30 THEN 'Fail in science' else '' END,
    CASE WHEN m.Hindi < 30 THEN 'Fail in Hindi' else '' END,
    CASE WHEN m.s_science < 30 THEN 'Fail in s_science' else '' END
)
AS Remark, 
CASE WHEN (m.math+m.English+m.science+m.hindi+m.s_science) < 150 THEN 'Fail' else 'Pass' END AS Result
from marks as m
join student as s
on m.st_id = s.st_id

Answer 2

您可以尝试使用 group_concat 来根据子查询

进行备注

  select s.*, t.Total, group_concat(t.remark), t.result  
  from  join student as s 
  INNER JOIN  (
    select s.st_id, m.math+m.English+m.science+m.hindi+m.s_science as Total,
        CASE WHEN m.math < 30 THEN 'Fail in Math' else '' END,
        CASE WHEN m.English < 30 THEN 'Fail in English' else '' END,
        CASE WHEN m.science < 30 THEN 'Fail in science' else '' END,
        CASE WHEN m.Hindi < 30 THEN 'Fail in Hindi' else '' END,
        CASE WHEN m.s_science < 30 THEN 'Fail in s_science' else '' END
    AS Remark,

    CASE WHEN (m.math+m.English+m.science+m.hindi+m.s_science) < 150 THEN 'Fail' else 'Pass' END AS Result
    from marks as m
    join student as s
    on m.st_id = s.st_id

    ) t  on t.st_id = s.st_id
  GROUP BY s.st_id

Answer 3

你可以用 concat():

select s.*, m.math+m.English+m.science+m.hindi+m.s_science as Total,
  trim(trailing ',' from  concat(
    CASE WHEN m.math < 30 THEN 'Fail in Math,' else '' END,
    CASE WHEN m.English < 30 THEN 'Fail in English,' else '' END,
    CASE WHEN m.science < 30 THEN 'Fail in science,' else '' END,
    CASE WHEN m.Hindi < 30 THEN 'Fail in Hindi,' else '' END,
    CASE WHEN m.s_science < 30 THEN 'Fail in s_science,' else '' END
  )) AS Remark,
CASE WHEN (m.math+m.English+m.science+m.hindi+m.s_science) < 150 THEN 'Fail' else 'Pass' END AS Result
from marks as m
join student as s
on m.st_id = s.st_id

Answer 4

像在单个案例中一样尝试这个

 select s.*, m.math+m.English+m.science+m.hindi+m.s_science as Total,
        CASE
     WHEN m.math < 30 THEN 'Fail in Math'
        WHEN m.English < 30 THEN 'Fail in English'
         WHEN m.science < 30 THEN 'Fail in science'  
         WHEN m.Hindi < 30 THEN 'Fail in Hindi' 
        WHEN m.s_science < 30 THEN 'Fail in s_science' 
     END
    AS Remark,
    CASE WHEN (m.math+m.English+m.science+m.hindi+m.s_science) < 150 THEN 'Fail' else 'Pass' END AS Result
    from marks as m
    join student as s
    on m.st_id = s.st_id

Answer 5

这可能只是评论部分对您的请求的评论，但对于评论来说太长了...

最好将数据库设计更改为三个表：

学生（student_id，姓名，...）
class（class_id，姓名，...）
student_class (student_id, class_id, 标记)

因此，您不必了解查询中的特定主题，也不必在每次删除或添加主题时更改所有查询。

select *
from student
join
(
  select
    sc.student_id,
    sum(sc.mark) as total,
    group_concat(case when sc.mark < 30 then c.name end
                 order by c.name separator ', ') as failed_classes,
    sum(sc.mark) < 150 as failed
  from student_class sc
  join class c on c.class_id = sc.class_id
  group by sc.student_id
) evaluated using (student_id)
order by student_id;

MySql 中的 WHEN 条件生成多列而不是一列？

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