Friend of a friend in PHP/MySQL?

Question

I have a social network similar to myspace/facebook. In my code you are either a person\'s friend or not a friend, so I show all actions from people you are friends with (in

Answer 1

The simple approach would be to do some kind of simple nested clause. So say you have a table with posts and the posters id, and a friends table, the first layer would be

SELECT post FROM posts JOIN friends 
  on post.userid = friends.friend_id 
  WHERE friend.id = 1 (user ID)

then to get a friends of friends

SELECT post FROM posts JOIN
   (SELECT DISTINCT friends_2.friend_id FROM friends AS friends_1 
        JOIN friends as friends_2 
        on friends_1.friend_id = friends_2.id where friends_1.id = 1) 
AS friends 
wHERE post.userid = friends.friend_id AND mainid = 1 (user ID)

You can repeat this nesting each time you want to add another layer of friend abstraction. The problem with this approach is that it would take a very long time to execute. For every time you add a layer of friend abstraction you are increasing the complexity by a power of n (where n is the number of rows in your table).

It is more likely that they are saving the viewable friends in a table somewhere, so lets make a new tabled called friends_web

user_id, friend_id, level

when a user friends someone, it adds that new friend into friends_web at a level of 0(since that friend is no people away) then adds that friends friends at a level of 1 (since its 1 friend away). In order to keep the table integrity you would also want to add the inverted record. To clarify if A adds B as a friend and C is a friend of B, the following two records would get added to our new table

A, C, 1
C, A, 1

since now A can see C and C can see A.

now when we want a query we just do

 SELECT post FROM posts 
  JOIN friends_web ON post.user_id = friends_web.friend_id 
  WHERE friends_web.user_id = user_id AND friends_web.level < 2 (or however deep you want to look)

by doing that you minimized your query complexity when doing post lookups while being able to look more then 1 layer deep into a friend web.

Sorry for the long winded response.