mysql查询之exists型子查询

### websites.id 和 access_log.site_id关联
mysql> select * from websites; 
+----+--------+
| id | name   |
+----+--------+
|  1 | google |
|  2 | taobao |
|  3 | baidu  |
+----+--------+


mysql> select * from access_log;
+----+---------+
| id | site_id |
+----+---------+
|  1 |       1 |
|  2 |       2 |
+----+---------+


### 查询有访问记录的网站信息
mysql> select * from websites a where exists (select 1 from access_log b where a.id=b.site_id);


+----+--------+
| id | name   |
+----+--------+
|  1 | google |
|  2 | taobao |
+----+--------+
2 rows in set


### 查询没有访问记录的网站信息
mysql> select * from websites a where not exists (select 1 from access_log b where a.id=b.site_id);


+----+-------+
| id | name  |
+----+-------+
|  3 | baidu |
+----+-------+
1 row in set