如何表示对称的多对多关系 [英] how to represent symmetric many to many relationship

问题描述

假设我有一个名为USER的实体，并且两个USER之间存在关系FRIENDSHIP
SO，因为我有一个表'USER'和一个关系表'FRIENDSHIP'

 
 
 
 
 USER 
 id firstName姓氏
 1 taher chhabrawala 
 2 john Dsouza 
 3 rahul singh 
 4 deepak patel 
 
 
 
友谊
 id id 
 1 2 
 1 3 
 1 4 
 4 1 
 4 3 
 
 
 
 

在上表中，我正在存储相同的信息两次，即taher是deepak和deepak的朋友是taher的朋友

有没有办法减少这种冗余嗯，你一定可以假定所有的友谊是对称的，只能存储一次友谊，但这意味着当你想查询所有的Taher的朋友时，你必须在任一列中查找他的身份证。

或者你可以有一个单独的关系ID表，然后与用户的一对多关系表。这有一个优点，如果有一天你会想要这个关系，那么它将允许多人关系，并且允许你添加关于关系的元数据（何时开始，谁建议，无论如何）。

 用户
 Id名称
 1 Taher 
 2 Deepak 
 
关系
 Id StartDate 
 1 2010-08-23 
 
 UserRelationship 
 RelationshipId UserId 
 1 1 
 1 2 
   
suppose i have a entity called USER and a relationship FRIENDSHIP exist between two USERs
SO for that i have a table 'USER' and a relationship table 'FRIENDSHIP'

   


USER
id  firstName  LastName
1    taher     chhabrawala
2    john      Dsouza
3    rahul     singh
4    deepak    patel

Friendship
id    id
1     2
1     3
1     4
4     1
4     3



In the above table i am storing the same information twice i.e "taher is a friend of deepak and deepak is a friend of taher"

Is there any way to reduce this redundancy ?
解决方案
Well, you could certainly just assume that all friendships are symmetric and store that friendship only once, but that would mean that when you want to query for all of Taher's friends, you have to look for his ID in either column.

Alternately you could have a separate table of relationship ID's, and then a one-to-many table of relationship to user.  This has the advantage that it would allow multi-person relationships if someday you want that, and would let you add meta-data about the relationship (when it started, who suggested it, whatever).
   User                
Id    Name          
1     Taher         
2     Deepak        

Relationship              
Id     StartDate           
1      2010-08-23

UserRelationship
RelationshipId UserId
1                1
1                2
On the other hand, on Facebook, for example, I can "friend" someone and they can decide not to friend me back.  If you don't have the "redundant" approach that you're using now, how will you represent that not-yet-reciprocal friendship? 

                        
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