select t.title,avg(s.salary) from
titles as t inner join salaries as s on t.emp_no = s.emp_no
group by title order by avg(s.salary)

首先做个表链接，然后将输出写出来，就是title 和 avg薪资
然后写条件用group by 对title 进行分组，用order by 对平均薪资分组
连起来就是以上语句