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 |
| liwu | Mac |
| lisi | xiaomi |
| maliu | nike |
+-------+-----------+
5 rows in set (0.00 sec)
查询
mysql> select * from tab_user u left join tab_order o on u.name=o.name and u.name='lisi';
+---------+------+--------+----------+------+--------+
| name | age | sex | addr | name | gname |
+---------+------+--------+----------+------+--------+
| daxin | 18 | male | beijing | NULL | NULL |
| mali | 28 | female | shandong | NULL | NULL |
| wangsan | 34 | male | beijing | NULL | NULL |
| lisi | 45 | male | liaoning | lisi | xiaomi |
| liwu | 58 | female | beijing | NULL | NULL |
| maoliu | 43 | male | anhui | NULL | NULL |
| zhouba | 62 | female | beijing | NULL | NULL |
+---------+------+--------+----------+------+--------+
7 rows in set (0.00 sec)
咋一看是不是很蒙圈，为什么已经限制了u.name='lisi'却查询结果还有其他人呢？如果换用where约束。
mysql> select * from tab_user u left join tab_order o on u.name=o.name where u.name='lisi';
+------+------+------+----------+------+--------+
| name | age | sex | addr | name | gname |
+------+------+------+----------+------+--------+
| lisi | 45 | male | liaoning | lisi | xiaomi |
+------+------+------+----------+------+--------+
1 row in set (0.00 sec)
结论
这次确实只有lisi了。那为什么第一个查询语句会与预期不符？回顾一下left join的定义，左边表会返回所有行，所以left join如果对左边表进行约束的话是不会生效的。但是，对left join的右边表添加条件的话是生效的！