分组
理解分组可以看如下一个例子，首先我们打印出products表如下
从上面的表中可以看出，每个vendor都有若干个产品，那么怎么一次统计每个vendor有多少个产品呢？ 这里就可以使用GROUP BY，如下
如果不使用group by，那么结果就是统计有多少行了：
分组过滤
分组过滤也就是多统计后的数据再进行过滤，如上面的例子中，统计出来了每一个vendor有多个产品，那么可以使用HAVING来过滤出 产品数量多于3个的vendor:
WHERE 语句可以喝HAVING语句一起灵和使用，如需要找出，有2种以上的产品价格大于10：
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