分组麻烦就用窗口函数

select 
    avg(salary)
from
    (select 
        max(salary) over() as ma,	# 直接获取到max
        min(salary) over() as mi,	# 直接获取到min
        salary
    from 
        salaries           as sa
    where 
        to_date ='9999-01-01')
where 
    salary <> ma and salary <> mi

# 2直接计算 
SELECT
	(SUM(salary) - MAX(salary) - MIN(salary)) / (COUNT(1) - 2)  # 去掉了最高最小
FROM salaries
WHERE to_date = '9999-01-01'