Friend Table with mysql
friend
=======
- id
- uid
- fid

Sample Data
===========
id  uid  fid
1    1    2
2    3    1

user
====
- id
- username

Sample Data
===========
id username
1  saturngod
2  snow
3  John

SELECT `user`.id,`user`.username FROM friend
INNER JOIN User
ON user.id = friend.uid
WHERE friend.fid = ( SELECT `id` FROM `User` WHERE `username`='saturngod')

UNION

SELECT `user`.id,`user`.username FROM friend
INNER JOIN User
ON user.id = friend.fid
WHERE friend.uid =  ( SELECT `id` FROM `User` WHERE `username`='saturngod')

SELECT u.id,u.username
  FROM user u
  INNER JOIN friend f ON (u.id = f.uid)
  WHERE u.username='saturngod'
UNION ALL
  SELECT u2.id,u2.username
  FROM user u
  INNER JOIN friend f ON (u.id = f.uid)
  INNER JOIN user u2 ON (u2.id = f.fid)
  WHERE u.username='saturngod'

SELECT
    u.id as user_id
    ,u.username
    ,u2.id as friend_id
    ,u2.username as friendname
  FROM user u
  INNER JOIN friend f ON (u.id = f.uid)
  LEFT JOIN user u2 ON (u2.id = f.fid)
  WHERE 'saturngod' = u.username

SELECT
    u.id as user_id
    ,u.username
    ,GROUP_CONCAT(u2.id) AS friend_ids
    ,GROUP_CONCAT(u2.username) as friendnames
  FROM user u
  INNER JOIN friend f ON (u.id = f.uid)
  LEFT JOIN user u2 ON (u2.id = f.fid)
  WHERE 'saturngod' = u.username  <<-- optional
  GROUP BY u.id

SELECT CASE WHEN user_id="$id" THEN friend_id ELSE user_id END AS friendID
FROM user_friend WHERE user_id="$id" OR friend_id="$id" order by friendID ASC