如何从SQL中的列中选择不同数量的值？

数据库包含注册表和学生表

注册表的列为：

CourseID, StudentID, CourseCode, Score, Year

CourseCode列包含课程代码，如CS-101、MS-202。每个学生ID注册到许多课程。我需要找到参加3门以上课程的学生的姓名和ID

我试过：

Select distinct 
    CourseRegistrations.S_ID, Students.FirstName
from 
    Students, CourseRegistrations
where 
    Students.StudentID = CourseRegistrations.StudentID 
group by
    CourseRegistrations.S_ID, Students.FirstName 
having 
    count(distinct CourseRegistrations.CourseCode) > 3

但这将显示文件的所有记录。

如果您使用GROUP BY，则不需要使用Distinct

不要使用老式连接

你应该试试：

Select CourseRegistrations.StudentID, count(CourseRegistrations.CourseCode)
from Students, CourseRegistrations
where Students.StudentID=CourseRegistrations.StudentID 
GROUP BY CourseRegistrations.StudentID
having count(CourseRegistrations.CourseCode)>3

-20多年前，在ANSI-92 SQL标准中，旧样式的逗号分隔表列表样式被正确的ANSI连接语法所取代，不鼓励使用它

Select CourseRegistrations.StudentID, count(CourseRegistrations.CourseCode)
from Students, CourseRegistrations
where Students.StudentID=CourseRegistrations.StudentID 
GROUP BY CourseRegistrations.StudentID
having count(CourseRegistrations.CourseCode)>3