记录遇到的MySQL的错误

1、'Operand should contain 1 column(s)'

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

错误：子查询中有两列，in条件后面有多个字段