Oracle EMPLOYEES表，以下哪个查询是有效的？

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 depe_id,join_date, SUM(salary) FROM employees GROUP BY dept_id:

C、SELECT dept_id, join_date, SUM(salary) FROM employees GROUP BY dept_id,join_date;

D、SELECT dept_ id,AVG (MAX (salary)) FROM employees GROUP BY dapt_id;

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