how to represent symmetric many to many relationship

Question

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\'

Answer 1

If you did decide to use the symmetric design then it might be useful to ensure that every friendship is always bidirectional. You can try this:

CREATE TABLE Friendship
 (UserId1 INT NOT NULL REFERENCES Users (UserId),
  UserId2 INT NOT NULL,
  PRIMARY KEY (UserId1, UserId2),
  FOREIGN KEY (UserId2, UserId1) REFERENCES Friendship (UserId1, UserId2));

INSERT INTO Friendship (UserId1, UserId2)
VALUES (1,2),(2,1);