select avg(salary) as avg_salary
from salaries a
where to_date='9999-01-01'
and a.salary not in (select max(salary) from salaries where to_date='9999-01-01')
and a.salary not in (select min(salary) from salaries where to_date='9999-01-01')

这道题的解题思路的本质依然是avg函数 但是需要用 where 做一个筛选