Is there an elegant way to store a dual relationship (i.e. user 1 and user 2 are friends)

Question

I\'ve run into the same problem in two different pieces of work this month:

Version 1: User 1 & User 2 are friends
Version 2: Axis 1 & Axis 2 when gr         


        

Answer 1

In SQL it's easy to implement the constraints to support your first approach:

CREATE TABLE MutualFriendship
(u1 VARCHAR(10) NOT NULL,
 u2 VARCHAR(10) NOT NULL,
 PRIMARY KEY (u1,u2),
 FOREIGN KEY (u2,u1) REFERENCES MutualFriendship (u1,u2));

INSERT INTO MutualFriendship VALUES
('Alice','Bob'),
('Bob','Alice');