考察left join
select a.last_name,a.first_name,c.dept_name from employees a left join dept_emp b on a.emp_no=b.emp_no left join departments c on b.dept_no=c.dept_no;
select a.last_name,a.first_name,c.dept_name from employees a left join dept_emp b on a.emp_no=b.emp_no left join departments c on b.dept_no=c.dept_no;