Question

1350. Students With Invalid Departments

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Description

Table: Departments

+---------------+---------+
| Column Name   | Type  |
+---------------+---------+
| id      | int   |
| name      | varchar |
+---------------+---------+
In SQL, 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   |
+---------------+---------+
In SQL, 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.

 

Find the id and the name of all students who are enrolled in departments that no longer exist.

Return the result table in any order.

The result format is in the following example.

 

Example 1:

Input: 
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       |
+------+----------+---------------+
Output: 
+------+----------+
| id   | name   |
+------+----------+
| 2  | John   |
| 7  | Daiana   |
| 4  | Jasmine  |
| 3  | Steve  |
+------+----------+
Explanation: 
John, Daiana, Steve, and Jasmine are enrolled in departments 14, 33, 74, and 77 respectively. department 14, 33, 74, and 77 do not exist in the Departments table.

Solutions

Solution 1: Subquery

We can directly use a subquery to find all students who are not in the Departments table.

# Write your MySQL query statement below
SELECT id, name
FROM Students
WHERE department_id NOT IN (SELECT id FROM Departments);

Solution 2: Left Join

We can also use a left join to join the Students table with the Departments table on the condition of Students.department_id = Departments.id, and then filter out the students whose Departments.id is NULL.

# Write your MySQL query statement below
SELECT s.id, s.name
FROM
  Students AS s
  LEFT JOIN Departments AS d ON s.department_id = d.id
WHERE d.id IS NULL;