方法一：

思路：用两个子查询找出最大工资和最小工资，再用not in去除最大工资和最小工资，最后记得限定时间为当前。

代码：

select
    avg(salary)
from
    salaries
where
    salary not in (
        select
            max(salary)
        FROM
            salaries
        where
            to_date = '9999-01-01'
        union all
        select
            min(salary)
        from
            salaries
        where
            to_date = '9999-01-01'
    )
    and to_date = '9999-01-01'