SELECT b.dept_name, a.emp_no, CONCAT(c.first_name, ' ', c.last_name) emp_name,
       a.from_date ,a.to_date
  FROM dept_manager a
 RIGHT JOIN departments b
    ON a.dept_no = b.dept_no
  LEFT JOIN employees c
    ON a.emp_no = c.emp_no
 WHERE SYSDATE() BETWEEN IFNULL(a.from_date, SYSDATE())
                     AND IFNULL(a.to_date, SYSDATE());