SQL GROUP BY 语句将具有相同值的行分组，它通常和聚合函数（COUNT，MAX，MIN，SUM，AVG）等一起使用。
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(s)
FROM table_name
WHERE condition
GROUP BY column_name(s)
ORDER BY column_name(s);
示例
简单应用
列出了每个国家/地区的客户数量：
SQL> SELECT country, COUNT(customer_id) FROM customers GROUP BY country;
执行后，输出如下：
+-----------+-------------------+ | country | COUNT(customer_id)| +-----------+-------------------+ | Argentina | 3 | | Austria | 2 | | Belgium | 2 | | Brazil | 9 | | Canada | 3 | | Denmark | 2 | +-----------+-------------------+
Join 联表分组
以下 SQL 语句列出了每个托运人发送的订单数：
SQL> SELECT s.shipper_name, COUNT(o.order_id) orders FROM orders o
LEFT JOIN shippers s ON o.shipper_id = s.shipper_id
GROUP BY s.shipper_name;
执行以上 SQL 输出结果如下：
+------------------+------------+ | shipper_name | orders | +------------------+------------+ | Federal Shipping | 68 | | Speedy Express | 54 | | United Package | 74 | +------------------+------------+
