select (sum(salary)-max_min.max_s-max_min.min_s)/(count()-2)
from salaries,
(
select max(salary) as max_s,min(salary)as min_s
from salaries where to_date='9999-01-01'
)as max_min
where to_date='9999-01-01'
写的有点麻烦了，以下方法是简单写法：
select (sum(salary)-max(salary)-min(salary))/(count()-2) from salaries where to_date='9999-01-01'