三个条件： salary 不等于 在职最大薪资； salary 不等于 在职最小薪资； 在职

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