Suppose an instance of this relationship in database where,
UID - $102$ has $2$ Friends $\{103 \ 104 \}$
Now if you try to map this instance to single table maintaining the integrity which looks like this :
User.UID |
User.Name |
Friend.Uid(Foreign key) |
102 |
a |
?? |
You can`t map anything because there will be redundancy and database will not allow that rather :
user.UID |
user.Name |
102 |
a |
103 |
b |
104 |
c |
friend.UID1 |
friend.UID2 |
102 |
103 |
102 |
104 |
Allowed and accepted and can be identified by combining both of these column.