select avg(salary) avg_salary
from salaries
where to_date='9999-01-01'
and salary not in((select max(salary) from salaries where to_date='9999-01-01'),
                  (select min(salary) from salaries where to_date='9999-01-01'))

/*
1、子查询语句错误
(select max(salary),min(salary) from salaries)
2、子查询的最大最小值，也需要限定9999-01-01条件
*/