考察多表链接 先 a和b链接 ，然后（a+b）和c链接

select a.last_name,a.first_name,b.dept_name from 
(select a.emp_no,birth_date,first_name,last_name,a.gender,a.hire_date,b.dept_no,b.from_date,b.to_date from employees a left join dept_emp b on a.emp_no=b.emp_no) a left join departments b on a.dept_no=b.dept_no