vend_id在两个表中都有，为了避免计算机无法识别，需要在前面加上表名 prod_id中含有0，说明含有vend_id的表是主表 两个表中都含有vend_id,通过和答案示例比较，我们发现Vendors是主表


select v.vend_id, ifnull(prod_id, 0) prod_id
from Vendors v
left join
(
    select vend_id, count(*) prod_id
    from Products group by vend_id
) p
on v.vend_id = p.vend_id
order by vend_id;