Self join many-to-many relationship - sql

Question

From sample data below, assuming Julie (1) has friends Adam, David, John (2, 3, 4).
Adam (2) has friends Julie, David, John (1, 3, 4).
ID Name
1 Julie
2 Adam
3 David
4 John
5 Sam
This make a self join and many-to-many relationship within ONE table.
In addition to the above problem, say Julie (1) added Sam (5) as friends, technically and practically speaking, Sam (5) is now friend of Julie (1) as well. This make things more complicated because the relationship bi-directional.
So I'm wondering:
How do I design the database?
How do I make a query that will return all friends of every users?
Thank you!

Answer 1

Example Data:
PEOPLE
PERS_ID PERS_NAME
1 Julie
2 Adam
3 David
4 John
5 Sam
FRIENDSHIPS
PERS_ID FRIEND_ID
1 2
1 3
1 4
2 3
2 4
Query:
select people.pers_id as person,
people.pers_name as person_name,
peoplef.pers_id as friend_id,
peoplef.pers_name as friend_name
from people
join friendships
on people.pers_id = friendships.pers_id
or people.pers_id = friendships.friend_id
join people peoplef
on (peoplef.pers_id = friendships.pers_id and
peoplef.pers_id <> people.pers_id)
or (peoplef.pers_id = friendships.friend_id and
peoplef.pers_id <> people.pers_id)
order by 2, 4
SQL Fiddle demo: http://sqlfiddle.com/#!2/97b41/6/0
This will work regardless of whether or not you record both directions on the friendships table.

Answer 2

Pretty much agree with the others. You need a link table. I'll give a bit more detail.. some examples of keys and indexes and the query you wanted (bi-directional).
CREATE TABLE dbo.tblUser
(
ID int identity(0,1),
name varchar(20)
CONSTRAINT PK_tblUser PRIMARY KEY (ID)
)
-- Many to many link table with FKs
CREATE TABLE dbo.tblFriend
(
ID1 int not null constraint FK_tblUser_ID1 foreign key references dbo.tblUser(ID),
ID2 int not null constraint FK_tblUser_ID2 foreign key references dbo.tblUser(ID)
CONSTRAINT PK_tblFriend PRIMARY KEY (ID1, ID2)
)
-- Add index (So you can get an index seek if using ID2)
CREATE INDEX IX_tblFriend_ID2 ON dbo.tblFriend (ID2)
-- Test data
INSERT INTO dbo.tblUser(name)
VALUES ('Julie'),('Adam'),('David'),('John'),('Sam');
Insert INTO dbo.tblFriend (ID1, ID2)
values(0, 1),(2, 0)
-- Get bi-directional friend to friend relationships
SELECT U1.Name as 'User1', U2.Name as 'User2' FROM dbo.tblFriend F
INNER JOIN dbo.tblUser U1 ON U1.ID = F.ID1
INNER JOIN dbo.tblUser U2 ON U2.ID = F.ID2
UNION
SELECT U2.Name as 'User1', U1.Name as 'User2' FROM dbo.tblFriend F
INNER JOIN dbo.tblUser U1 ON U1.ID = F.ID1
INNER JOIN dbo.tblUser U2 ON U2.ID = F.ID2
ORDER BY User1, User2

Answer 3

One approach could be that you create second table that stores the person and friend ids. In this scenario, consider the following tables.
CREATE TABLE User
(
id int auto_increment primary key,
name varchar(20)
);
CREATE TABLE Friend
(
user_id int ,
friend_id int
);
INSERT INTO User
(name)
VALUES
('Julie'),
('Adam'),
('David'),
('John'),
('Sam');
Insert INTO Friend
(user_id, friend_id)
values(1, 5),
(3, 1);
Now the Friend table will store the user_id and his/her friend_id. For getting the list of friends for a particular user, you can search the id matching in either of these two columns. Below are sample queries.
-- Get Friends of Julie
select 1 AS user_id, IF(user_id = 1, friend_id, user_id) AS friend_id
FROM Friend
WHERE user_id=1 OR friend_id=1;
-- Get Friends of David
select 3 AS user_id, IF(user_id = 3, friend_id, user_id) AS friend_id
FROM Friend
WHERE user_id=3 OR friend_id=3
I hope you get idea with this and can play around.

Answer 4

I tried whatever you written in your query:
declare #table table
(
id int,
name varchar(40)
)
insert into #table values
(1, 'Julie'),
(2, 'Adam'),
(3, 'David'),
(4, 'John'),
(5, 'Sam')
select
t1.name ,
t2.name as friend
from #table t1, #table t2 where t1.id <> t2.id
and t1.id in (1,2) and t2.id <> 5
order by t1.id