[Leetcode] Employees Whose Manager Left the Company

문제 링크

정답

SELECT E.employee_id
FROM Employees E
LEFT JOIN Employees M
ON E.manager_id = M.employee_id
WHERE E.manager_id IS NOT NULL AND M.employee_id IS NULL AND E.salary < 30000
ORDER BY E.employee_id;

✅ Point

• Self-Join
• LEFT JOIN + NULL 을 통해 존재하지 않음을 찾기