sql exists() 函数

exists 运算符用于判断查询子句是否有记录，如果有一条或多条记录存在返回 true，否则返回 false。

1. exists() 语法

select column_name(s)
from table_name
where exists
(select column_name from table_name where condition);

样本数据库

在本教程中，我们将使用 yapf 样本数据库。

下面是选自 "websites" 表的数据：

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | google       | https://www.google.cm/    | 1     | usa     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | cn      |
| 3  | 硕编程      | http://www.yapf.com/  | 4689  | cn      |
| 4  | 微博          | http://weibo.com/         | 20    | cn      |
| 5  | facebook     | https://www.facebook.com/ | 3     | usa     |
+----+---------------+---------------------------+-------+---------+

下面是 "access_log" 网站访问记录表的数据：

+-----+---------+-------+------------+
| aid | site_id | count | date       |
+-----+---------+-------+------------+
|   1 |       1 |    45 | 2016-05-10 |
|   2 |       3 |   100 | 2016-05-13 |
|   3 |       1 |   230 | 2016-05-14 |
|   4 |       2 |    10 | 2016-05-14 |
|   5 |       5 |   205 | 2016-05-14 |
|   6 |       4 |    13 | 2016-05-15 |
|   7 |       3 |   220 | 2016-05-15 |
|   8 |       5 |   545 | 2016-05-16 |
|   9 |       3 |   201 | 2016-05-17 |
+-----+---------+-------+------------+

2. exists() 范例

现在我们想要查找总访问量(count 字段)大于 200 的网站是否存在。

select websites.name, websites.country 
from websites 
where exists (select count from access_log where websites.id = access_log.site_id and count > 200);

执行结果：
+---------------+-------------+
|   name        | country     |
+---------------+-------------+
| google        |    usa      |
| 硕编程       |     cn      |
| facebook      |    usa      |
+---------------+-------------+

exists 可以与 not 一同使用，查找出不符合查询语句的记录：

select websites.name, websites.country
from websites 
where not exists (select count from access_log where websites.id = access_log.site_id and count > 200);

执行结果：
+---------------+-------------+
|   name        |    country  |
+---------------+-------------+
|   淘宝         |     cn      |
|   微博         |     cn      |
+---------------+-------------+