직무별 평균 재직기간을 출력하세요.SELECT AVG(sub.working_period) AS avg_work, t.titleFROM ( SELECT timestampdiff( year, MIN(s.from_date), IF(MAX(s.to_date) = '9999-01-01', NOW() ,MAX(s.to_date)) ) AS working_period, emp_no FROM salaries AS s GROUP BY emp_no) as subJOIN titles AS t ON sub.emp_no = t.emp_noJOIN employees AS e ON e.emp_n..