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      SQL GROUP BY

      SQL GROUP BY 语句

      GROUP BY 语句可结合一些聚合函数来使用

      GROUP BY 语句

      GROUP BY 语句用于结合聚合函数,根据一个或多个列对结果集进行分组。

      SQL GROUP BY 语法

      SELECT column_name, aggregate_function(column_name)
      FROM table_name
      WHERE column_name operator value
      GROUP BY column_name;
      

      演示数据库

      在本教程中,我们将使用 axihe 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  | 阿西河前端教程      | http://www.axihe.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:

      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)
      
      

      GROUP BY 简单应用

      统计 access_log 各个 site_id 的访问量:

      SELECT site_id, SUM(access_log.count) AS nums
      FROM access_log GROUP BY site_id;
      

      SQL GROUP BY 多表连接

      下面的 SQL 语句统计有记录的网站的记录数量:

      SELECT Websites.name,COUNT(access_log.aid) AS nums FROM access_log
      LEFT JOIN Websites
      ON access_log.site_id=Websites.id
      GROUP BY Websites.name;
      
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