1.根据两个表的 emp_no inner join 成一个新表;
2.然后根据 title 分组,求其平均值,过滤出相应字段,再根据平均薪水升序排序

select title,avg(salary) avg_salary from
(select * from titles) a
inner join
(select * from salaries) b
on a.emp_no = b.emp_no
group by title
order by avg_salary