Oracle EMPLOYEES表，以下哪个查询是有效的？ A-SELECT dept_id,MAX (AVG (salary)) FROM employees GROUP BY dept_id; B-SELECT dept_ id,AVG (MAX (salary)) FROM employees GROUP BY dapt_id; C-SELECT depe_id,join_date, SUM(salary) FROM employees GROUP BY dept_id: D-SELECT dept_id, join_date, SUM(salary) FROM employees GROUP BY dept_id,join_date;