Answer by TheTXI for Facebook database design?

Keep a friend table that holds the UserID and then the UserID of the friend (we will call it FriendID). Both columns would be foreign keys back to the Users table.

Somewhat useful example:

Table Name: UserColumns:    UserID PK    EmailAddress    Password    Gender    DOB    LocationTableName: FriendsColumns:    UserID PK FK    FriendID PK FK    (This table features a composite primary key made up of the two foreign      keys, both pointing back to the user table. One ID will point to the     logged in user, the other ID will point to the individual friend     of that user)

Example Usage:

Table User--------------UserID EmailAddress Password Gender DOB      Location------------------------------------------------------1      bob@bob.com  bobbie   M      1/1/2009 New York City2      jon@jon.com  jonathan M      2/2/2008 Los Angeles3      joe@joe.com  joseph   M      1/2/2007 PittsburghTable Friends---------------UserID FriendID----------------1      21      32      3

This will show that Bob is friends with both Jon and Joe and that Jon is also friends with Joe. In this example we will assume that friendship is always two ways, so you would not need a row in the table such as (2,1) or (3,2) because they are already represented in the other direction. For examples where friendship or other relations aren't explicitly two way, you would need to also have those rows to indicate the two-way relationship.