Question

我有两张桌子，一张是学生的成绩，一张是他们正在上的课程。

表1：结果

|student | course | date | grade | examiner | PK:id| 
|:-------|:-------|:-----|:------|:--------:|:-----:
|20122222|INFA    | null |   2   |   TC     |  1   | 
|20123333|INFA    | null |   8   |   TC     |  2   | 
|20122222|INFA    | null |   4   |   VB     |  3   | 
|20122222|INFA    | null |   6   |   VB     |  4   | 
|20123333|INFB    | null |   10  |   null   |  5   | 
|20124444|INFB    | null |   4   |   VB     |  6   | 
|20121234|INFA    | null |   8   |   VB     |  7   | 
|20121234|INFC    | null |   8   |   GV     |  8   |

PK代表主键，我也遗漏了日期，因为它们无关紧要

表2：课程

|PK:code|     name      |coordinator|
|:------|:-------------:|:---------:|
|INFA   |Programming    |VB         |
|INFB   |Databases      |VB         |           
|INFC   |WebApplications|BD         |
|INFD   |OO Programming |null       |

现在的目标是获得按降序排序的每个课程名称的最高，最低和平均成绩。现在我已经尝试了几件事，但我似乎无法正确输出。输出应为：

|name           |highest|lowest|average|
|:-------------:|:------|:-----|:------|
|WebApplications|8      |8     |8.0    |
|Databases      |10     |4     |7.0    |
|Programming    |8      |2     |5.6    |

但我不能将它们排在课程名称上

Answer 1

我猜查询应该如下所示：

SELECT min(grade) AS lowest, max(grade) AS highest, avg(grade) AS average 
FROM results 
GROUP BY course 
ORDER BY average DESC.

如果您希望在同一查询中包含课程名称，请加入结果：

SELECT course.name, min(grade) AS lowest, max(grade) AS highest, avg(grade) AS average 
FROM results
JOIN course on results.course = course.code 
GROUP BY course     
ORDER BY average DESC

Answer 2

如果你有一个没有结果的课程并且想要显示它，我会使用左连接课程来获得结果，然后使用基本的聚合功能。我假设平均降序。

SELECT c.name, max(R.grade) highest, min(R.grade) lowest, avg(R.grade) mAverage
FROM Course C
LEFT JOIN Results R
 on C.Code = R.Course
WHERE C.Coordinator is not null
--Not sure if SQL server allows you to group by columns not in select; I know mySQL will if group by extensions are enabled.
GROUP BY C.Code, C.Name
ORDER BY mAverage DESC

我需要按降序SQL选择某些名称的最大值，最小值和平均值

2 个答案: