方法一:在联结三个表时,使用两次left join

select e.last_name, e.first_name, d.dept_name
from employees e left join dept_emp de on e.emp_no = de.emp_no
                 left join departments d on de.dept_no = d.dept_no 

方法二:先联结deparments和dept_emp成为一个新表b,再将其与employees表联结(注意:此方法必须要注意列名的命名,否则容易报错)

select e.last_name, e.first_name, b.ddept_name
from employees e left join (select de.emp_no deemp_no, d.dept_name ddept_name
                           from departments d left join dept_emp de
                           on d.dept_no = de.dept_no) b
on e.emp_no = b.deemp_no