那么怎么可以體現二者區別呢？
很簡單，連接時對應字段一致。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;
這樣執行select后結果分別是8和9
