那么怎么可以体现二者区别呢？
很简单，连接时对应字段一致。也就是：
select a.user_name,a.over,b.over
... from user1 a left join user2 b on a.user_name=b.user_name
... union
... select b.user_name,a.over,b.over
... from user1 a right join user2 b on a.user_name=b.user_name;
这样执行select后结果分别是8和9
