How do i display friends of a user in mysql? - php

Question

I am creating a friend list module.
I have 2 tables: users table and friends table. I want the program to display the friends of a user without displaying the user as one of his/her own friend.
My table looks like
Users table
---------- ---------- ---------
user_id | username | password
-------------------------------
1 | elexis | *******
-------------------------------
2 | rooney | *******
-------------------------------
3 | wayne | *******
-------------------------------
4 | June | *******
-------------------------------
Friends table
---------- ----------
user_id | friend_id
---------------------
1 | 3
-----------
2 | 1
-----------
2 | 4
-----------
1 | 2
Expected output of user_id 1 (who is elexis) should be
User_id friend_id Username
1 3 Wayne
2 1 Rooney
My code is thus:
SELECT t1.user_id,t1.friend_id, t2.username
FROM friends AS t1
LEFT JOIN users AS t2
ON (t1.user_id = t2.user_id)
OR (t1.friend_id = t2.user_id)
WHERE t1.user_id = 1
The output I'm getting is:
User_id Friend_id Username
1 5. Elexis
1 1 Elexis
What am I getting wrong?

Answer 1

Join Users to a query that contains distinct pairs of user with id = 1 and each pair:
SELECT f.*, u.username
FROM Users u
INNER JOIN (
SELECT user_id, friend_id FROM Friends WHERE user_id = 1
UNION
SELECT friend_id, user_id FROM Friends WHERE friend_id = 1
) f ON f.friend_id = u.user_id;
Or, if you want to pass the user parameter only once:
SELECT f.*, u.username
FROM Users u
INNER JOIN (
SELECT user_id, friend_id FROM Friends
UNION
SELECT friend_id, user_id FROM Friends
) f ON f.friend_id = u.user_id
WHERE f.user_id = 1
See the demo.

Answer 2

You need to join on Friend_id and select user_id from Friends table.
SELECT f.user_id,f.friend_id,u.username
FROM friends f JOIN users u
ON f.user_id=f.friend_id
This gives you output:
user_id | friend_id | username
1 | 3 | Wayne
2 | 1 | Elexis

Answer 3

You can union two qieroes which give you the result
CREATE TABLE friend
(`user_id` int, `friend_id` int)
;
INSERT INTO friend
(`user_id`, `friend_id`)
VALUES
(1, 3),
(2, 1),
(2, 4),
(1, 2)
;
CREATE TABLE users
(`user_id` int, `username` varchar(6), `password` varchar(7))
;
INSERT INTO users
(`user_id`, `username`, `password`)
VALUES
(1, 'elexis', '*******'),
(2, 'rooney', '*******'),
(3, 'wayne', '*******'),
(4, 'June', '*******')
;
SELECT f.*, u. username FROM friend f JOIN users u ON f.friend_id = u.user_id WHERE f.user_id = 1 AND f.`friend_id` NOT IN (SELECT user_id FROM friend WHERE friend_id = 1)
UNION
SELECT f.*, u.username FROM friend f JOIN users u ON f.user_id = u.user_id WHERE friend_id = 1
user_id | friend_id | username
------: | --------: | :-------
1 | 3 | wayne
2 | 1 | rooney
db<>fiddle here