题解 |不使用 join ，仅单表查询
获取所有员工当前的manager
http://www.nowcoder.com/practice/e50d92b8673a440ebdf3a517b5b37d62
select e.emp_no , (select d.emp_no from dept_manager d where d.dept_no = e.dept_no) manager
from dept_emp e
where emp_no not in (select emp_no from dept_manager)
from dept_emp e
where emp_no not in (select emp_no from dept_manager)
