Leetcode1350. 院系无效的学生（简单）

题目
院系表: Departments

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| name          | varchar |
+---------------+---------+

id 是该表的主键
该表包含一所大学每个院系的 id 信息

学生表: Students

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| id            | int     |
| name          | varchar |
| department_id | int     |
+---------------+---------+

id 是该表的主键
该表包含一所大学每个学生的 id 和他/她就读的院系信息

写一条 SQL 语句以查询那些所在院系不存在的学生的 id 和姓名

可以以任何顺序返回结果

下面是返回结果格式的例子

Departments 表:

+------+--------------------------+
| id   | name                     |
+------+--------------------------+
| 1    | Electrical Engineering   |
| 7    | Computer Engineering     |
| 13   | Bussiness Administration |
+------+--------------------------+

Students 表:

+------+----------+---------------+
| id   | name     | department_id |
+------+----------+---------------+
| 23   | Alice    | 1             |
| 1    | Bob      | 7             |
| 5    | Jennifer | 13            |
| 2    | John     | 14            |
| 4    | Jasmine  | 77            |
| 3    | Steve    | 74            |
| 6    | Luis     | 1             |
| 8    | Jonathan | 7             |
| 7    | Daiana   | 33            |
| 11   | Madelynn | 1             |
+------+----------+---------------+

结果表:

+------+----------+
| id   | name     |
+------+----------+
| 2    | John     |
| 7    | Daiana   |
| 4    | Jasmine  |
| 3    | Steve    |
+------+----------+

John, Daiana, Steve 和 Jasmine 所在的院系分别是 14, 33, 74 和 77， 其中 14, 33, 74 和 77 并不存在于院系表

解答
选出student表中department_id不在departments表中的记录

select S.id, S.name
from Students as S
where S.department_id not in (select D.id
from Departments as D)

左连接也可以

SELECT Students.id, Students.name 
FROM Students LEFT JOIN Departments ON Students.department_id = Departments.id 
WHERE Departments.id is null;