select em.emp_no,de.emp_no as manager
FROM dept_emp as em left join dept_manager as de
on em.dept_no=de.dept_no
where em.emp_no!=de.emp_no

构建dept_emp as em left join dept_manager as de并查集, 得到员工与经理的复合表,从中选取em.emp_no和de.emp_no不相等的(即非经理的员工)