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 web3 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  | 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：

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) AS nums
FROM access_log GROUP BY site_id;

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 |
+---------+------+
5 rows in set (0.00 sec)

SQL GROUP BY 多表连接

现在我们想要查找每个送货员配送的订单数目。

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 FROM access_log
LEFT JOIN Websites
ON access_log.site_id=Websites.id
GROUP BY Websites.name;

mysql> 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;
+----------+------+
| name     | nums |
+----------+------+
| Facebook |    2 |
| Google   |    2 |
| 微博     |    1 |
| 淘宝     |    1 |
| 芝麻教程 |    3 |
+----------+------+
5 rows in set (0.01 sec)