题解 | #列出供应商及其可供产品的数量#

select Vendors.vend_id,cast(sum(if(Products.prod_id is null,0,1)) AS UNSIGNED) as prod_id
from Vendors
left join Products on Vendors.vend_id = Products.vend_id
group by Vendors.vend_id
order by Vendors.vend_id;

主要是通过供应商去找哪些家供应了货物的，另：在找出来后通过判定可知道有没有值，然后算出来的值为double，需要使用cast函数进行转换一下就好