select avg(salary) from salaries
where salary != (select max(salary) from salaries where to_date = '9999-01-01') 
and salary != (select min(salary) from salaries where to_date = '9999-01-01')
and to_date = '9999-01-01';

这个题目没描述的很清楚，首先，你要排除在职的最大，最小薪资

其次你要筛选的是在职人的平均薪资，不是排除在职的其他人的。