查找所有已经分配部门的员工的last_name和first_name以及dept_no

/* 方法一 笛卡尔积
select t2.last_name, t2.first_name, t1.dept_no
from dept_emp as t1, employees as t2
where t1.emp_no = t2.emp_no
*/

-- 方法二 left join
select t2.last_name, t2.first_name, t1.dept_no
from dept_emp as t1 left join employees as t2
on t1.emp_no = t2.emp_no