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。

语法

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

示例

"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  | 菜鸟教程      | http://www.runoob.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     |
+----+---------------+---------------------------+-------+---------+

"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 |
+-----+---------+-------+------------+

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;

输出结果：

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

+---------+--------+
| site_id |  nums  |
+---------+--------+
|    1    | 275    |
|    2    |  10    |
|    3    | 521    |
|    4    |  13    |
|    5    | 750    |
+---------+--------+

GROUP BY 多表连接

统计所有网站的访问的记录数：

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;

输出结果：


+--------------+--------+
| name        |  nums  |
+-------------+--------+
| Facebook    |   2    |
| Google      |   2    |
| 微博        |   1    |
| 淘宝        |   1    |
| 菜鸟教程     |  3    |
+--------------+--------+