1978. Employees Whose Manager Left the Company

Description

Table: Employees

+-------------+----------+
| Column Name | Type     |
+-------------+----------+
| employee_id | int      |
| name        | varchar  |
| manager_id  | int      |
| salary      | int      |
+-------------+----------+
In SQL, employee_id is the primary key for this table.
This table contains information about the employees, their salary, and the ID of their manager. Some employees do not have a manager (manager_id is null).

Find the IDs of the employees whose salary is strictly less than $30000 and whose manager left the company. When a manager leaves the company, their information is deleted from the Employees table, but the reports still have their manager_id set to the manager that left.

Return the result table ordered by employee_id.

The result format is in the following example.

Example 1:

Input:  
Employees table:
+-------------+-----------+------------+--------+
| employee_id | name      | manager_id | salary |
+-------------+-----------+------------+--------+
| 3           | Mila      | 9          | 60301  |
| 12          | Antonella | null       | 31000  |
| 13          | Emery     | null       | 67084  |
| 1           | Kalel     | 11         | 21241  |
| 9           | Mikaela   | null       | 50937  |
| 11          | Joziah    | 6          | 28485  |
+-------------+-----------+------------+--------+
Output: 
+-------------+
| employee_id |
+-------------+
| 11          |
+-------------+

Explanation: 
The employees with a salary less than $30000 are 1 (Kalel) and 11 (Joziah).
Kalel's manager is employee 11, who is still in the company (Joziah).
Joziah's manager is employee 6, who left the company because there is no row for employee 6 as it was deleted.

Solutions

Solution 1: Left Join

Thinking

We want employees paid under $30000$ whose manager id is set but no longer in the table. A self left-join that fails to match the manager row detects a departed manager.

We can use a left join to connect the employee table with itself, and then filter out the employees whose salary is less than $30000$ and have a superior manager who has left the company.

MySQL

# Write your MySQL query statement below
SELECT e1.employee_id
FROM
    Employees AS e1
    LEFT JOIN Employees AS e2 ON e1.manager_id = e2.employee_id
WHERE e1.salary < 30000 AND e1.manager_id IS NOT NULL AND e2.employee_id IS NULL
ORDER BY 1;

Solution 2: Subquery

Thinking

The join can be replaced by $\texttt{NOT IN}$: the manager id is absent from $\texttt{employee\_id}$. The meaning is the same and the query is shorter.

We can also use a subquery to first find all the managers who have left the company, and then find the employees whose salary is less than $30000$ and whose superior manager is not in the list of managers who have left the company.