1、创建分组 group by

SELECT vend_id, COUNT(*) AS num_prods FROM products
GROUP BY vend_id;

在where字句之后，在order by字句之前。

2、过滤分组

HAVING 过滤分组，where过滤行

SELECT cust_id,COUNT(*) AS orders1 FROM orders
GROUP BY cust_id
HAVING COUNT(*) >=2;

它列出具有两个以上产品且其价格大于等于4的供应商： 

SELECT vend_id,COUNT(*) AS num_prods FROM products
WHERE prod_price >=4
GROUP BY vend_id
HAVING COUNT(*) >=2;

SELECT子句顺序

mysql之分组