数据库SQL实践54：查找排除当前最大、最小salary之后的员工的平均工资avg_salary

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);