SELECT AVG(f.salary)
  FROM (SELECT a.dept_no, a.dept_name,
               COUNT(*) cnt, SUM(c.salary) salary
          FROM departments a, dept_emp b, salaries c
         WHERE a.dept_no = b.dept_no
           AND b.emp_no = c.emp_no
           AND SYSDATE() BETWEEN b.from_date AND b.to_date
           AND SYSDATE() BETWEEN c.from_date AND c.to_date
         GROUP BY a.dept_no, a.dept_name
       ) f;