先找到最大值和最小值（用开窗函数排序），剔除后取平均。

select avg(t1.salary)
from (select salary, dense_rank() over (order by salary) as minSalary, dense_rank() over (order by salary desc ) as maxSalary
      from salaries
      where to_date = '9999-01-01') as t1
where t1.minSalary <> 1 and t1.maxSalary <> 1;