sql having 子句

having 子句可以筛选分组后的各组数据，而 where 关键字无法与聚合函数一起使用。

1. having 语法

select column_name, aggregate_function(column_name)
from table_name
where column_name operator value
group by column_name
having aggregate_function(column_name) operator value;

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     |
| 7  | stackoverflow | http://stackoverflow.com/ |     0 | ind     |
+----+---------------+---------------------------+-------+---------+

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. sql having 范例

现在我们想要查找总访问量大于 200 的网站。

我们使用下面的 sql 语句：

select websites.name, sum(access_log.count) as nums from (access_log
inner join websites
on access_log.site_id=websites.id)
group by websites.name
having sum(access_log.count) > 200;

执行结果：
+-----------+----------+
|   name    |   nums   |
+-----------+----------+
|  facebook |    750   |
|  google   |    275   |
|  硕编程  |    521   |
+-----------+----------+

现在我们想要查找总访问量大于 200 的网站，并且 alexa 排名小于 200。

我们在 sql 语句中增加一个普通的 where 子句：

select websites.name, sum(access_log.count) as nums from websites
inner join access_log
on websites.id=access_log.site_id
where websites.alexa < 200 
group by websites.name
having sum(access_log.count) > 200;

执行结果：
+-----------+----------+
|   name    |   nums   |
+-----------+----------+
|  facebook |    750   |
|  google   |    275   |
+-----------+----------+