SELECT a.prod_name,b.order_num
FROM Products a
left join OrderItems b on a.prod_id=b.prod_id
union
SELECT a.prod_name,b.order_num
FROM Products a
left join OrderItems b on a.prod_id=b.prod_id
where a.prod_id is null
order by prod_name

# orderby针对select后面的内容排序的，如果最后一行是a.prod_name则会报错，因为prod_id为空的时候，a.prod_name关联不到。