SQL中join操作后面的on 和 where 的区别

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。