SQL GROUP BY 语句

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_nameWHERE column_name operator valueGROUP BY column_name;演示数据库在本教程中，我们将使用 RUNOOB 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 | k88.net | http:





//www.k88.net/ | 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：实例SELECT site_id, SUM(access_log.count) ASnums FROM access_log GROUP BY site_id;执行以上 SQL 输出结果如下：SQL GROUP BY 多表连接下面的 SQL 语句统计所有网站的访问的记录数：实例SELECT Websites.name,COUNT(access_log.aid) AS nums FROMaccess_logLEFT JOIN WebsitesON access_log.site_id=Websites.idGROUP BY Websites.name;执行以上 SQL 输出结果如下：