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