一、MySQL JOIN 分类
JOIN 按照功能大致分为如下三类：
INNER JOIN（内连接）：取得两个表中存在连接匹配关系的记录。
LEFT JOIN（左连接）：取得左表（table1）完全记录，即是右表（table2）并无对应匹配记录。
RIGHT JOIN（右连接）：与 LEFT JOIN 相反，取得右表（table2）完全记录，即是左表（table1）并无匹配对应记录。
二、图解关系
INNER JOIN：用于取得两个表中存在连接匹配关系的记录。
mysql> select * from a inner join score on a.sn=score.sn;
+----+---------+-------+----+-------+-------+
| id | name | sn | id | sn | score |
+----+---------+-------+----+-------+-------+
| 1 | mashen | 10086 | 1 | 10086 | 90 |
| 2 | haishen | 10087 | 2 | 10087 | 59 |
| 3 | haoge | 10088 | 3 | 10088 | 77 |
+----+---------+-------+----+-------+-------+
3 rows in set (0.24 sec)
mysql> select * from a join score on a.sn=score.sn;
+----+---------+-------+----+-------+-------+
| id | name | sn | id | sn | score |
+----+---------+-------+----+-------+-------+
| 1 | mashen | 10086 | 1 | 10086 | 90 |
| 2 | haishen | 10087 | 2 | 10087 | 59 |
| 3 | haoge | 10088 | 3 | 10088 | 77 |
+----+---------+-------+----+-------+-------+
3 rows in set (0.00 sec)
Inner join
产生的结果集中，是A和B的交集。
当2个关联表字段相同时候 也可以用using(sn),这样using 里的字段显示一次.....
mysql> select * from a join score using(sn);
+-------+----+---------+----+-------+
| sn | id | name | id | score |
+-------+----+---------+----+-------+
| 10086 | 1 | mashen | 1 | 90 |
| 10087 | 2 | haishen | 2 | 59 |
| 10088 | 3 | haoge | 3 | 77 |
+-------+----+---------+----+-------+
LEFT JOIN:产生表A的完全集，而B表中匹配的则有值，没有匹配的则以null值取代
mysql> select * from a left join score on a.sn=score.sn;
+----+-----------+-------+------+-------+-------+
| id | name | sn | id | sn | score |
+----+-----------+-------+------+-------+-------+
| 1 | mashen | 10086 | 1 | 10086 | 90 |
| 2 | haishen | 10087 | 2 | 10087 | 59 |
| 3 | haoge | 10088 | 3 | 10088 | 77 |
| 8 | left join | 11122 | NULL | NULL | NULL |
+----+-----------+-------+------+-------+-------+
4 rows in set (0.00 sec)
##产生在A表中有而在B表中没有的集合，在业务求新增的时候经常使用到的语法：
mysql> select * from a left join score on a.sn=score.sn where score.id is null;
+----+-----------+-------+------+------+-------+
| id | name | sn | id | sn | score |
+----+-----------+-------+------+------+-------+
| 8 | left join | 11122 | NULL | NULL | NULL |
+----+-----------+-------+------+------+-------+
1 row in set (0.01 sec)
##产生在b表中有而在a表中没有的集合，就是INNER JOIN
mysql> select * from a left join score on a.sn=score.sn where score.id is not null
+----+---------+-------+------+-------+-------+
| id | name | sn | id | sn | score |
+----+---------+-------+------+-------+-------+
| 1 | mashen | 10086 | 1 | 10086 | 90 |
| 2 | haishen | 10087 | 2 | 10087 | 59 |
| 3 | haoge | 10088 | 3 | 10088 | 77 |
+----+---------+-------+------+-------+-------+
RIGHT JOIN:产生表B的完全集，而A表中匹配的则有值，没有匹配的则以null值取代,与left join 相反.
mysql> select * from a right join score on a.sn=score.sn
-> ;
+------+---------+-------+----+-------+-------+
| id | name | sn | id | sn | score |
+------+---------+-------+----+-------+-------+
| 1 | mashen | 10086 | 1 | 10086 | 90 |
| 2 | haishen | 10087 | 2 | 10087 | 59 |
| 3 | haoge | 10088 | 3 | 10088 | 77 |
| NULL | NULL | NULL | 4 | 10089 | 77 |
| NULL | NULL | NULL | 5 | 10090 | 70 |
+------+---------+-------+----+-------+-------+
5 rows in set (0.00 sec)
##产生在b表中有而在a表中没有的集合,没有匹配显示null
mysql> select * from a right join score on a.sn=score.sn where a.id is null;
+------+------+------+----+-------+-------+
| id | name | sn | id | sn | score |
+------+------+------+----+-------+-------+
| NULL | NULL | NULL | 4 | 10089 | 77 |
| NULL | NULL | NULL | 5 | 10090 | 70 |
+------+------+------+----+-------+-------+