https://www.nowcoder.com/practice/e50d92b8673a440ebdf3a517b5b37d62?tpId=82&tags=&title=&difficulty=0&judgeStatus=0&rp=1

SELECT
  de.emp_no,
  dm.emp_no AS manager_no 
FROM
  dept_emp de
LEFT JOIN dept_manager dm ON de.dept_no = dm.dept_no 
WHERE
  de.to_date = '9999-01-01'
AND dm.to_date = '9999-01-01'
AND de.emp_no <> dm.emp_no;