单向关系

Question

我希望在我的表mysql表'friends'中找到单向关系

Name    Friend
John    Mary
Krish   Paula
Debbie  John
Paula   Krish
Mary    John

我想列出所有没有配对（B - A）的配对（A - B） 示例：查询应选择第4行（Debbie - John），因为（John - Debbie）不存在。

Answer 1

Select f1.* from Friends as f1
left join Friends as f2 on f1.Name = f2.Friend and f1.Friend = f2.Name
where f2.Name is null

Sql Fiddle

Answer 2

您也可以使用 greatest() and least()

Select 
greatest(Name,Friend),least(Name,Friend)
from Friends
group by greatest(Name,Friend),least(Name,Friend)
having count(1)=1

<强> SQL FIDDLE

Answer 3

这是解决问题的查询。

select * from friends where name not in (select a.name from friends a, friends b where a.friend=b.name and a.name=b.friend);

Answer 4

Select Name, Friend from Friends as f1
left join Friends as f2 on f1.Name = f2.Friend AND f1.Friend = f2.Name