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On Mar 30, 1:05 am, "Aloha Kakuikanu" <aloha.kakuik..._at_yahoo.com>
wrote:
> On Mar 29, 2:15 am, mlef..._at_yahoo.com wrote:
>
>
>
> > Hi-
> > I've been banging my head over how to represent this theoretical
> > example: in a group of people, trying to keep track of who is friends
> > with whom. I'm assuming that if person A is friends with person B,
> > then the reverse is also true. However if B is friends with C, then A
> > is not necessarily friends with C. What is the best way to represent
> > this to ensure best integrity and at the same time make the structure
> > simple and efficient to query. Here is my best go at is:
>
> > TABLE 1 - people (PrimaryKey: person_id)
> > person_id
> > person_name
>
> > TABLE 2 - friendships (PrimaryKey: friendship_id, pair_id, secondary
> > unique key: friendship_id, person_id )
> > friendship_id ## this id will start at 1 and increment for each
> > 'pair' of associated recs added (so will be the same number for two
> > records)
> > pair_id ## restricted value: 1, 2--> purpose is to only
> > allow two values to be stored for each 'friendship pair'. Keeps
> > database structure intact.
> > person_id ## id of one person in a pair of friends
>
> Friendship is symmetric but not transitive binary relation. You have 2
> options to represent it:
> 1. "Normalized form". Only one tuple per each relationship. E.g.
> <person1=A, person2=B> means that A is friend with B and B is friend
> with A. You can enforce relationship uniqueness by tuple constraint:
>
> person1 < personB
>
> 2. "Symmetrically closed" relation. Two tuples per each relationship.
> E.g. <person1=A, person2=B> means that A is friend with B, and
> <person1=B, person2=A> means that B is friend with A. You can enforce
> symmetric closure with complex constraint.
Thanks for the input. I was trying to see if i could model it without using person1, person2 tuples since I thought this would introduce directionality into the relationship and make it more roundabout to design queries. However maybe the two-record "Symmetrically closed relation" option you mention is pretty much equivalent? Hmmm... not sure. Received on Thu Mar 29 2007 - 21:00:10 CDT