简单题，考察连接以及分组
统计出当前各个title类型对应的员工当前薪水对应的平均工资
https://www.nowcoder.com/practice/c8652e9e5a354b879e2a244200f1eaae
select a.title, AVG(b.salary) as 'avg(s.salary)' from titles as a left join salaries as b on a.emp_no = b.emp_no group by a.title order by 'avg(s.salary)';
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