院系表: Departmentssql
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| id | int |
| name | varchar |
+---------------+---------+
id 是该表的主键
该表包含一所大学每一个院系的 id 信息
网络
学生表: Studentsui
+---------------+---------+
| 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一个学生的 id 和他/她就读的院系信息
code
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leetcode
能够以任何顺序返回结果it
下面是返回结果格式的例子io
Departments 表:
+------+--------------------------+
| id | name |
+------+--------------------------+
| 1 | Electrical Engineering |
| 7 | Computer Engineering |
| 13 | Bussiness Administration |
+------+--------------------------+asm
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 |
+------+----------+---------------+class
结果表:
+------+----------+
| id | name |
+------+----------+
| 2 | John |
| 7 | Daiana |
| 4 | Jasmine |
| 3 | Steve |
+------+----------+方法
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)
连接:https://leetcode-cn.com/problems/students-with-invalid-departments
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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,直接经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: