Microsoft SQL Server 简单分组
示例
订单表
按特定列分组时，仅返回此列的唯一值。
SELECT customerId FROM orders GROUP BY customerId;
返回值：
聚合函数count()适用于每个组，而不适用于完整的表：
SELECT customerId,
COUNT(productId) as numberOfProducts,
sum(price) as totalPrice
FROM orders
GROUP BY customerId;返回值：