Question

我希望在我的SQL查询中有一个switch case，这样当group by不对任何元素进行分组时我不想聚合，否则我想要。这可能。

我的查询是这样的：

select count(1),AVG(student_mark) ,case when Count(1)=1 then student_subjectid else null end from Students
group by student_id

我收到此错误列'student_subjectid'在选择列表中无效，因为它不包含在聚合函数或GROUP BY子句中。

提前致谢..

Answer 1

SELECT
  student_id,
  COUNT(*) AS MarkCount,
  AVG(student_mark) AS student_mark,
  CASE COUNT(*) WHEN 1 THEN MIN(student_subjectid) END AS student_subjectid
FROM Students
GROUP BY student_id

Answer 2

为什么世界会让你复杂化？

select count(1), AVG(Student_mark) Student_mark
from Students
group by student_id

如果只有一个student_mark，它也是SUM，AVG，MIN和MAX - 所以只需继续使用聚合！

<小时/>

修改

最符合您要求的数据集通常不会有意义。实现这一目标的方法是合并（联合）两个不同的结果

select
    numRecords,
    Student_mark,
    case when numRecords = 1 then student_subjectid end    # else is implicitly NULL
from
(
select
    count(1) AS numRecords,
    AVG(Student_mark) Student_mark,
    min(student_subjectid) as student_subjectid
from Students
group by student_id
) x

在聚合查询中切换案例

2 个答案:

修改