SQL HAVING

HAVING 子句

在 SQL 中增加 HAVING 子句原因是,WHERE 关键字无法与聚合函数一起使用。

HAVING 子句可以让我们筛选分组后的各组数据。

SQL HAVING 语法

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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;

演示数据库

在本教程中,我们将使用 web3  样本数据库。

下面是选自 "Websites" 表的数据:

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+----+--------------+---------------------------+-------+---------+
| id | name | url | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1 | Google | https://www.google.cm/ | 1 | USA |
| 2 | 淘宝 | https://www.taobao.com/ | 13 | CN |
| 3 | 芝麻教程 | http://www.web3.xin/ | 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:

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mysql> SELECT * FROM access_log;
+-----+---------+-------+------------+
| 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 |
+-----+---------+-------+------------+
9 rows in set (0.00 sec)

SQL HAVING 实例

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:

实例

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SELECT Websites.name, Websites.url, 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;
执行以上 SQL 输出结果如下:
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mysql> select websites.name, websites.url, 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;
+----------+---------------------------+------+
| name | url | nums |
+----------+---------------------------+------+
| Facebook | https://www.facebook.com/ | 750 |
| Google | https://www.google.cm/ | 275 |
| 芝麻教程 | http://www.web3.xin/ | 521 |
+----------+---------------------------+------+
3 rows in set (0.01 sec)

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:

实例

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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;
执行以上 SQL 输出结果如下:
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mysql> select websites.name, websites.url, sum(access_log.count) 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;
+----------+---------------------------+-----------------------+
| name | url | sum(access_log.count) |
+----------+---------------------------+-----------------------+
| Facebook | https://www.facebook.com/ | 750 |
| Google | https://www.google.cm/ | 275 |
+----------+---------------------------+-----------------------+
2 rows in set (0.01 sec)

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