题解 | #返回产品名称和每一项产品的总订单数#

select prod_name, count(order_num) orders
from Products a left join OrderItems b
on a.prod_id = b.prod_id
group by prod_name
order by prod_name;








-- 下面的不行，会显示none
select prod_name, orders
from Products a left join (
    select count(order_num) orders,prod_id
    from OrderItems
    group by prod_id
) b on a.prod_id = b.prod_id
order by prod_name;




select count(order_num) orders
from OrderItems
group by prod_id;