思路:
先选出最大工资,再选出最小工资,最终排除最大和最小工资,求平均。(但题目要求排除当前最大工资和当前最小工资,似乎有bug)
select avg(salary) as avg_salary from salaries
where to_date = '9999-01-01'
and salary not in (select max(salary) from salaries)
and salary not in (select min(salary) from salaries);