#该题要求使用outer join,但是由于mysql不支持outer join,所以只能使用left join +right join实现全联结
(select p.prod_name, oi.order_num
from Products p
left join OrderItems oi
on p.prod_id = oi.prod_id)
union
(select p.prod_name, oi.order_num
from Products p
right join OrderItems oi
on p.prod_id = oi.prod_id) 
order by prod_name
#UNION 操作符会自动合并两个结果集的列名，所以不用给两个select语句起别名