题解 | #列出供应商及其可供产品的数量#

select v.vend_id, count(p.prod_id) as prod_id
from Vendors v
left join Products p on v.vend_id = p.vend_id
group by v.vend_id
order by v.vend_id;

使用左外连接，然后按vendors表的id分组，因为是以左表作为基准