查找各个部门当前(to_date='9999-01-01')领导当前薪水详情以及其对应部门编号dept_no

CREATE TABLE `dept_manager` (
`dept_no` char(4) NOT NULL,
`emp_no` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`dept_no`));

CREATE TABLE `salaries` (
`emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));

这道题是真的很郁闷，以下是两个我最初的答案，都显示不对：

select sa.*, de.dept_no 
from salaries as sa, dept_manager as de
where sa.emp_no = de.emp_no
and sa.to_date = de.to_date
and sa.from_date = de.from_date
and de.to_date='9999-01-01';

select sa.*, de.dept_no 
from salaries as sa, dept_manager as de
where sa.emp_no = de.emp_no
and sa.to_date = de.to_date
and de.to_date='9999-01-01';

好烦啊~ 求大神知道，以下是通过的SQL语句：

select sa.*, de.dept_no 
from salaries as sa, dept_manager as de
where sa.emp_no = de.emp_no
and de.to_date = '9999-01-01'
and sa.to_date = '9999-01-01';