1978. Employees Whose Manager Left the Company.sql

/*
Question 1978. Employees Whose Manager Left the Company
Link: https://leetcode.com/problems/employees-whose-manager-left-the-company/description/?envType=study-plan-v2&envId=top-sql-50

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.
*/

SELECT e1.employee_id
FROM Employees AS e1
WHERE
    e1.salary < 30000 AND e1.manager_id NOT IN (
        SELECT e2.employee_id
        FROM Employees AS e2
    )
ORDER BY e1.employee_id ASC