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 CodeBaoku 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  | 编程宝库      | http://www.codebaoku.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      |
| 编程宝库       |     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      |
+---------------+-------------+

UCASE() 函数把字段的值转换为大写。SQL UCASE() 语法：SELECT UCASE(column_name) FROM table_name; 用于 SQL Server 的语法：SELECT UPPER(column_name) FROM table_name。