题解 | 返回产品名称和每一项产品的总订单数
select prod_name, cast(sum(判断) as unsigned) orders from ( select prod_name, if (order_num is not null, 1, 0) 判断 from Products pro left join OrderItems ori on pro.prod_id = ori.prod_id ) a group by prod_name order by prod_name asc