题解 | 列出供应商及其可供产品的数量

select
    a.vend_id vend_id,
    cast(sum(判断) as unsigned) prod_id
from
    (
        select
            ven.vend_id,
            if (prod_id is null, 0, 1) 判断
        from
            Vendors ven
            left join Products pro on ven.vend_id = pro.vend_id
    ) a
group by
    a.vend_id
order by
    vend_id asc