仅当值不在另一个表中时，选择查询才能从表列中获取值

Question

如果只有值不在另一个表中，请有人建议我使用SQL Server Select查询从表列中获取值，这就是我想要做的事情：

SELECT t.TeacherID
FROM Teacher t ...... (Who are not in Class c, c.TeacherID Column)

Answer 1

select teacherID from Teacher
Except
select teacherID from Class

或者您也可以选择：

select teacherID from Teacher where TeacherId not in(select teacherID from Class)

Answer 2

不 的教师有课程：

SELECT *
FROM   teacher
WHERE  NOT EXISTS (
         SELECT *
         FROM   class
         WHERE  class.teacherid = teacher.teacherid
       )

如果您想要反向（查找 做 的教师有课程），那么您所要做的就是扭转条件：删除NOT ！

Answer 3

你也可以使用LEFT JOIN WHERE没有为教师iD分配课程

`SELECT teacher.ID `
`FROM   teacher LEFT JOIN Class ON teacher.ID =  Class.ID`
`WHERE   Class.ID IS NOT NULL`