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Formatted question description: https://leetcode.ca/all/1350.html
1350. Students With Invalid Departments
Level
Easy
Description
Table: Departments
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| id | int |
| name | varchar |
+---------------+---------+
id is the primary key of this table.
The table has information about the id of each department of a university.
Table: Students
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| id | int |
| name | varchar |
| department_id | int |
+---------------+---------+
id is the primary key of this table.
The table has information about the id of each student at a university and the id of the department he/she studies at.
Write an SQL query to find the id and the name of all students who are enrolled in departments that no longer exists.
Return the result table in any order.
The query result format is in the following example:
Departments table:
+------+--------------------------+
| id | name |
+------+--------------------------+
| 1 | Electrical Engineering |
| 7 | Computer Engineering |
| 13 | Bussiness Administration |
+------+--------------------------+
Students table:
+------+----------+---------------+
| 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 |
+------+----------+---------------+
Result table:
+------+----------+
| id | name |
+------+----------+
| 2 | John |
| 7 | Daiana |
| 4 | Jasmine |
| 3 | Steve |
+------+----------+
John, Daiana, Steve and Jasmine are enrolled in departments 14, 33, 74 and 77 respectively. department 14, 33, 74 and 77 doesn't exist in the Departments table.
Solution
For each entry in Students
table, if department_id
is not in id
of Departments
table, then the department with department_id
no longer exists. Therefore, select the fields id
and name
from Students
, and select the entries such that department_id
are not in id
field of Departments
table.
# Write your MySQL query statement below
select id, name from Students where department_id not in (select id from Departments);