建立分组：

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

vend_id是Vendor_id的缩写：供应商id，products是一个存储商品信息的表。

通过GROUP BY语句，我们可以按照供应商分组，然后组内计数。

SELECT a.prod_id,a.prod_name,a.prod_price,a.prod_desc,b.num_prods
FROM products a
join 
(
SELECT vend_id,count(*) as num_prods
FROM products
group by vend_id
) b
on a.vend_id=b.vend_id
where num_prods>2

查询提供2个以上产品的供应商的产品。