#outer join 等于left join union right join

思路： select A.prod_name,count(A.order_num) orders from (select prod_name,order_num from Products left join OrderItems on Products.prod_id = OrderItems.prod_id union select prod_name,order_num from OrderItems right join Products on Products.prod_id = OrderItems.prod_id) A group by A.prod_name order by A.prod_name