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
来源:力扣(LeetCode)
链接:https://leetcode-cn.com/problems/students-with-invalid-departments
著作权归领扣网络所有。商业转载请联系官方授权,非商业转载请注明出处。
审题: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
思考:查询不存在的院系的学生,直接通过排除存在的剩下的就是不存在的。
解题:
方法一:
SELECT Students.id, Students.name
FROM Students LEFT JOIN Departments ON Students.department_id = Departments.id
WHERE ISNULL(Departments.id);
方法二:
SELECT id, name
FROM Students
WHERE NOT department_id IN (
SELECT id
FROM Departments
);
知识点: