Facebook database design?

Question

I have always wondered how Facebook designed the friend <-> user relation.

I figure the user table is something like this:

user_email PK
user_id PK
password 

I figure the table with user's data (sex, age etc connected via user email I would assume).

How does it connect all the friends to this user?

Something like this?

user_id
friend_id_1
friend_id_2
friend_id_3
friend_id_N 

Probably not. Because the number of users is unknown and will expand.

Answer 1

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: User
Columns:
    UserID PK
    EmailAddress
    Password
    Gender
    DOB
    Location

TableName: Friends
Columns:
    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      [email protected]  bobbie   M      1/1/2009 New York City
2      [email protected]  jonathan M      2/2/2008 Los Angeles
3      [email protected]  joseph   M      1/2/2007 Pittsburgh

Table Friends
---------------
UserID FriendID
----------------
1      2
1      3
2      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.