join 链接查询
inner join 、left join 、right join 关系和区别
inner join(内链接）只返回两表中链接字段相等的记录
left join(左链接) 返回包括左表中的所有记录和右表中 与左表联结字段相等 的记录.(两表中链接字段相等的记录，以及左表所有剩下的记录)
right join(右链接) 返回包括右表中的所有记录和左表中 与右表联结字段相等 的记录.(两表中链接字段相等的记录，以及左表所有剩下的记录,效果类似和 left join 左右两表交换一样)
现有两表如下：
user:
+----+-------+---------+
| id | name | addr |
+----+-------+---------+
| 1 | tom | street1 |
| 2 | DIVA | street2 |
| 3 | simis | street2 |
+----+-------+---------+
record:
+----+------+-----------+---------+
| id | msg | date | user_id |
+----+------+-----------+---------+
| 1 | bai | 1233 | 1 |
| 2 | wds | 132233 | 2 |
| 3 | west | asf132233 | 1 |
| 4 | hfd | 34fw | NULL |
+----+------+-----------+---------+使用 inner join 连接查询
mysql> select * from user u inner join record r on u.id = r.user_id;查询结果中，只返回链接字段相等记录：
+----+------+---------+----+------+-----------+---------+
| id | name | addr | id | msg | date | user_id |
+----+------+---------+----+------+-----------+---------+
| 1 | tom | street1 | 1 | bai | 1233 | 1 |
| 1 | tom | street1 | 3 | west | asf132233 | 1 |
| 2 | DIVA | street2 | 2 | wds | 132233 | 2 |
+----+------+---------+----+------+-----------+---------+使用 left join：
mysql> select * from user u left join record r on u.id = r.user_id;查询结果比使用 inner join 多返回了左表 user 的剩余数据记录，该记录和右表链接全为 NULL。
+----+-------+---------+------+------+-----------+---------+
| id | name | addr | id | msg | date | user_id |
+----+-------+---------+------+------+-----------+---------+
| 1 | tom | street1 | 1 | bai | 1233 | 1 |
| 2 | DIVA | street2 | 2 | wds | 132233 | 2 |
| 1 | tom | street1 | 3 | west | asf132233 | 1 |
| 3 | simis | street2 | NULL | NULL | NULL | NULL |
+----+-------+---------+------+------+-----------+---------+使用 right join:
mysql> select * from user u right join record r on u.id = r.user_id;查询结果中返回了所有右表记录，并且还有左表中链接字段相等的记录。
+------+------+---------+----+------+-----------+---------+
| id | name | addr | id | msg | date | user_id |
+------+------+---------+----+------+-----------+---------+
| 1 | tom | street1 | 1 | bai | 1233 | 1 |
| 1 | tom | street1 | 3 | west | asf132233 | 1 |
| 2 | DIVA | street2 | 2 | wds | 132233 | 2 |
| NULL | NULL | NULL | 4 | hfd | 34fw | NULL |
+------+------+---------+----+------+-----------+---------+