单个表看成多个表叫做自连接

#自连接
#查询 员工名和上级名称

SELECT
e.employee_id,e.last_name,l.`employee_id`,l.`last_name`
FROM
employees e,employees l
WHERE e.employee_id=l.manager_id
GROUP BY l.`employee_id`,l.`last_name`;