Reflexive Relation.

Hi, I would like advice on how to most efficiently design a reflexive relation.

I have 2 use cases for that:

1) table A have a 1-n relation with itself. For example I want to modelise A1 as an row and to notify that A1 is compatible with A2 / A3 and A4. But this also implies that A2 is compatible with A1 (but it doesn't imply that A2 and A" are compatible). So the relation A1-A2 is identical to the relation A2-A1. The main query used on this would be to answer the 2 questions "What is An compatible with? " "Is An compatible with Am ?". I would like to avoid having to play my queries both way and checking if the *reversed* relation exists as well in the database before inserting it...

2) table Segment define a connection between two points. A segment is not oriented and is defined by 2 extremities. In the following example, Segment 1 and Segment 2 are somehow identical and violating the uniqness...

Segment 1
----------
Extremity_1 = A
Extremity_2 = B

Segment 2
----------
Extremity_1 = B
Extremity_2 = A

I would like to implement this the best way to abstract this complexity from all queries.

Thanks for your help =)