join关键字的作用是将多个表按一定的条件联合起来，从而可以实现从多个表中获取数据。
join的常见用法有join、left join 、right join 、full join。
on 和 where 的区别：
on 表示在 join 前进行条件筛选，然后再进行join操作。
where 表示在join操作完了再做过滤。
示例：
现在有两张表t1 和 t2，表里面的数据如下：
> select * from t1;
+------+------+
| id | age |
+------+------+
| 1 | 20 |
| 2 | 21 |
| 3 | 22 |
| 4 | 23 |
| 5 | 24 |
+------+------+
> select * from t2;
+------+------+
| id | name |
+------+------+
| 1 | Lee |
| 2 | Bob |
| 3 | Kate |
| 4 | Tony |
+------+------+
思考一下下面的sql 执行后的结果，有几条数据：
1）select * from t1 left join t2 on t1.id=t2.id and t1.id=1;
2）select * from t1 left join t2 on t1.id=t2.id and t2.id=1;
3）select * from t1 left join t2 on t1.id=t2.id where t1.id=1;
4）select * from t1 left join t2 on t1.id=t2.id where t2.id=1;
思考5秒钟......
1 seconds...
2 seconds...
3 seconds...
4 seconds...
5 seconds...
答案是：5、5、1、1
实际输出结果如下：
1） select * from t1 left join t2 on t1.id=t2.id and t1.id=1;
+------+------+------+------+
| id | age | id | name |
+------+------+------+------+
| 1 | 20 | 1 | Lee |
| 2 | 21 | NULL | NULL |
| 3 | 22 | NULL | NULL |
| 4 | 23 | NULL | NULL |
| 5 | 24 | NULL | NULL |
+------+------+------+------+
2） select * from t1 left join t2 on t1.id=t2.id and t2.id=1;
+------+------+------+------+
| id | age | id | name |
+------+------+------+------+
| 1 | 20 | 1 | Lee |
| 2 | 21 | NULL | NULL |
| 3 | 22 | NULL | NULL |
| 4 | 23 | NULL | NULL |
| 5 | 24 | NULL | NULL |
+------+------+------+------+
3） select * from t1 left join t2 on t1.id=t2.id where t1.id=1;
+------+------+------+------+
| id | age | id | name |
+------+------+------+------+
| 1 | 20 | 1 | Lee |
+------+------+------+------+
4） select * from t1 left join t2 on t1.id=t2.id where t2.id=1;
+------+------+------+------+
| id | age | id | name |
+------+------+------+------+
| 1 | 20 | 1 | Lee |
+------+------+------+------+
注意一点：
left join 时，左表的数据是完整的，on 条件会筛选需要关联的列，无论能否关联上，左表都是完整的。比如下面的例子：
> select * from t1 left join t2 on t1.id=t2.id and t2.id=10;
+------+------+------+------+
| id | age | id | name |
+------+------+------+------+
| 1 | 20 | NULL | NULL |
| 2 | 21 | NULL | NULL |
| 3 | 22 | NULL | NULL |
| 4 | 23 | NULL | NULL |
| 5 | 24 | NULL | NULL |
+------+------+------+------+
同理，right join 也是一样的。