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、題目需求：已知某學校組織籃球比賽，現在有兩張表class_info 和 match_info，需要通過查詢獲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：利用左連接查詢，同時根據實際情況，match_info需要連接class_info兩次（可以多次使用left join連接），才能將match_info中的host_id和guest_id轉化為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、總結：
犯錯一：表的別名重復問題
【注意】在進行多次連接同一個表時，一定不能存在表的重名。解決方法為給重名的表設置不同的表名：例 table_a as t_a
