mysql join 多个条件_数据库left join中多个条件需要特殊注意之处

left join查询会从左表那里返回所有的行，即使在右表中没有匹配的行。查询中on条件只有一个，因此不存在特殊注意之处。但是当我们on条件如果存在多个时候会出现一些与我们预期不符的查询结果。

用户表

mysql> select * from tab_user;

+---------+------+--------+----------+

| name | age | sex | addr |

+---------+------+--------+----------+

| daxin | 18 | male | beijing |

| mali | 28 | female | shandong |

| wangsan | 34 | male | beijing |

| lisi | 45 | male | liaoning |

| liwu | 58 | female | beijing |

| maoliu | 43 | male | anhui |

| zhouba | 62 | female | beijing |

+---------+------+--------+----------+

7 rows in set (0.00 sec)

订单表

mysql> select * from tab_order;

+-------+-----------+

| name | gname |

+-------+-----------+

| daxin | Smartisan |

| mali | iPhone |