sql - 仅当值不在另一个表中时，选择查询以从表列中获取值

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：

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

或者你也可以去：

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

Answer 2

不上课的老师：

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

如果你想做相反的事情（找到有课的老师），那么你所要做的就是反转条件：删除NOT!

Answer 3

您也可以使用 LEFT JOIN WHERE 没有为教师 ID 分配班级

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