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;