属性名重名，用表名区分
获取所有部门当前manager的当前薪水情况，给出dept_no, emp_no以及salary
http://www.nowcoder.com/questionTerminal/4c8b4a10ca5b44189e411107e1d8bec1
select dept_manager.dept_no, dept_manager.emp_no, salaries.salary from dept_manager, salaries where salaries.emp_no=dept_manager.emp_no and dept_manager.to_date='9999-01-01' and salaries.to_date='9999-01-01';