class_info
| id | class_name |
| 2 | s204 |
| 5 | s205 |
| 1 | s207 |
| 7 | s203 |
match_info
| id | host_id | guest_id | match_time | match_result |
| 1 | 2 | 1 | 2018-12-10 | 45:62 |
| 2 | 2 | 7 | 2018-12-16 | 55:50 |
| 3 | ||||
| 4 | ||||
| 5 |
1、题目需求：已知某学校组织篮球比赛，现在有两张表class_info 和 match_info，需要通过查询获得如下格式表格信息：
| host | guest | match_time | match_result |
| s204 | s207 | 2018-12-10 | 45:62 |
| s204 | s203 | 2018-12-16 | 55:50 |
2、分析：利用左连接查询，同时根据实际情况，match_info需要连接class_info两次（可以多次使用left join连接），才能将match_info中的host_id和guest_id转化为class_name。
3、操作：
select c.class_name as host,d.class_name as guest, m.match_time,m.match_result
from match_info as m left join class_info as c
on m.host_id=c.id
left join class_info as d on m.guest_id=d.id;
4、总结：
犯错一：表的别名重复问题
【注意】在进行多次连接同一个表时，一定不能存在表的重名。解决方法为给重名的表设置不同的表名：例 table_a as t_a
