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