有歧义的部分，必须指定表名

select salaries.emp_no, salary, salaries.from_date, salaries.to_date, dept_no
from salaries, dept_manager
where dept_manager.to_date='9999-01-01' and salaries.to_date='9999-01-01'
and salaries.emp_no=dept_manager.emp_no
order by salaries.emp_no asc;