SQL 服务器：从一个表中获取所有数据，但条件在第二个表列

Question

我有 3 张表Student 、 Classes和StudentClasses 。

学生表：

班级表：

StudentClasses表：

我正在尝试的是

select 
    c.ClassID, sc.StudentID, Title, 
    ClassFrom as ClassDate, ClassTo as ClassTime,
    Duration, Type as SessionType, sc.Status as StatusJoin
from 
    Classes c 
left join 
    StudentClasses sc on sc.ClassID = c.ClassID 
where 
    sc.StudentID = '66919287-d63d-4b30-931f-30532b68c2f1' or 
    sc.StudentID IS NULL

这将返回：

我认为这是正确的，但是当我将其更改为：

where sc.StudentID = '5a22f025-ae64-49b8-9782-32bc2f1ccef6'

这返回：

这应该返回所有 4 个具有学生 ID 和加入日期 null 的班级

这是问题：我希望这里是所有课程，而不是一些学生加入或不加入，那么应该有 Studentid 和 JoiningDate 否则这些列可以是 null

Answer 1

在 ON 子句中使用您的条件而不是 Where

 select c.ClassID,sc.StudentID,Title,ClassFrom as ClassDate,ClassTo as 
       ClassTime,Duration,Type as SessionType,sc.Status as StatusJoin
       from Classes c 
       left join StudentClasses sc on sc.ClassID = c.ClassID 
       and (sc.StudentID = '66919287-d63d-4b30-931f-30532b68c2f1' or 
       sc.StudentID IS NULL)

Answer 2

select c.ClassID,sc.StudentID,Title,ClassFrom as ClassDate,ClassTo as ClassTime,Duration,Type as SessionType,sc.Status as StatusJoin from Classes c left join StudentClasses sc on sc.ClassID = c.ClassID

也会有帮助！！！