left join 关键字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 的条件只有一个，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！反之，right join 同理！