sql having 子句

having 子句可以篩選分組后的各組數據，而 where 關鍵字無法與聚合函數一起使用。

1. having 語法

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;

樣本數據庫

在本教程中，我們將使用 yapf 樣本數據庫。

下面是選自 "websites" 表的數據：

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | google       | https://www.google.cm/    | 1     | usa     |
| 2  | 淘寶          | https://www.taobao.com/   | 13    | cn      |
| 3  | 碩編程      | http://www.090948.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" 網站訪問記錄表的數據：

+-----+---------+-------+------------+
| 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

現在我們想要查找總訪問量大于 200 的網站。

我們使用下面的 sql 語句：

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;

執行結果：
+-----------+----------+
|   name    |   nums   |
+-----------+----------+
|  facebook |    750   |
|  google   |    275   |
|  碩編程  |    521   |
+-----------+----------+

現在我們想要查找總訪問量大于 200 的網站，并且 alexa 排名小于 200。

我們在 sql 語句中增加一個普通的 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;

執行結果：
+-----------+----------+
|   name    |   nums   |
+-----------+----------+
|  facebook |    750   |
|  google   |    275   |
+-----------+----------+