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    |
| 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！