sql – 自我加入多对多关系

根据以下样本数据,假设Julie(1)有朋友Adam,David,John(2,3,4).
亚当(2)有朋友朱莉,大卫,约翰(1,4).

ID  Name
1   Julie
2   Adam
3   David
4   John
5   Sam

这使得一个表中的自连接和多对多关系成为可能.

除了上述问题之外,Julie(1)还说Sam(5)是朋友,从技术上和实际上讲,Sam(5)现在也是Julie(1)的朋友.这使得事情变得更加复杂,因为这种关系是双向的.

所以我想知道：

>我如何设计数据库？
>如何进行将返回每个用户的所有朋友的查询？

谢谢！

解决方法

示例数据：

PEOPLE

PERS_ID     PERS_NAME
1           Julie
2           Adam
3           David
4           John
5           Sam

FRIENDSHIPS

PERS_ID     FRIEND_ID
1           2
1           3
1           4
2           3
2           4

查询：

select people.pers_id    as person,people.pers_name  as person_name,peoplef.pers_id   as friend_id,peoplef.pers_name as friend_name
  from people
  join friendships
    on people.pers_id = friendships.pers_id
    or people.pers_id = friendships.friend_id
  join people peoplef
    on (peoplef.pers_id = friendships.pers_id and
       peoplef.pers_id <> people.pers_id)
    or (peoplef.pers_id = friendships.friend_id and
       peoplef.pers_id <> people.pers_id)
 order by 2,4

sql小提琴演示：http://sqlfiddle.com/#!2/97b41/6/0

无论您是否在友谊表上记录两个方向,这都将有效.