step1:取出需要的字段； step2：关联、聚合运算

select title
      ,avg(salary) 
  from (
		select title
		      ,emp_no
		  from titles
       )p1

 inner join
       (
		select salary
		      ,emp_no
		  from salaries
       )p2
    on p1.emp_no = p2.emp_no
 group by title
  order by avg(salary) asc