<?php ------>Created By SirDarknight<--------
-- TABLES

drop table if exists users;
create table users
(
user_id int unsigned not null auto_increment primary key,
username varbinary(32) unique not null
)
engine=innodb;

drop table if exists user_friends;
create table user_friends
(
user_id int unsigned not null,
friend_user_id int unsigned not null,
created_date datetime not null,
primary key (user_id, friend_user_id) -- note clustered composite PK (innodb only)
)
engine=innodb;

-- TRIGGERS

delimiter #

create trigger user_friends_before_ins_trig before insert on user_friends
for each row
begin
  set new.created_date = now();
end#

delimiter ;

-- STORED PROCEDURES

drop procedure if exists insert_user_friend;

delimiter #

create procedure insert_user_friend
(
in p_user_id int unsigned,
-->in p_friend_user_id int<--- unsigned
)
proc_main:begin ------>Created By SirDarknight

   if p_user_id = p_friend_user_id then
     leave proc_main;
   end if;

   insert into user_friends (user_id, friend_user_id) values (p_user_id, p_friend_user_id);

end proc_main #

delimiter ;

drop procedure if exists list_user_friends;

delimiter #
create procedure list_user_friends
(
in p_user_id int unsigned
)
proc_main:begin

   select
     u.*,
     uf.created_date,
     date_format(uf.created_date, '%e-%b-%Y') as created_date_fmt
   from
     user_friends uf
   inner join users u on uf.friend_user_id = u.user_id
   where
     uf.user_id = p_user_id
   order by
    u.username;

end proc_main #

delimiter ;

-- TEST DATA

insert into users (username) values ('f00'),('bar'),('alpha'),('delta'),('omega'),('theta');

call insert_user_friend(1,2);
call insert_user_friend(1,3);
call insert_user_friend(1,4);
call insert_user_friend(1,1); -- oops

call insert_user_friend(2,1);
call insert_user_friend(2,5);

call insert_user_friend(4,1);

call insert_user_friend(6,1);

-- TESTING (call these sproc from your php !)

call list_user_friends(1);