MySQL获取尚未成为朋友的用户（自我排除）

Question

This is the continuation of my question here

So basically, I want to fetch users who are not my friends. However, if a user has just signed up and he has no friends yet, he should be able to see all users not his friends.

Please check my sql fiddle here

 Users 
 id    username   
 1     kyle
 2     jane
 3     jim
 4     carla

 Friends
 id    username  friend
 1     kyle      jane
 2     jane      kyle
 3     kyle      jim
 4     jim       kyle

When I query Kyle's not his friends:

SELECT * FROM users AS u 
                    WHERE u.username NOT IN 
                    (
                        SELECT f.username FROM friends as f
                        WHERE f.username = 'kyle'
                        OR f.friend = 'kyle'
                    )

It's displaying all users not his friends. However, when I query Carla's, she is included in the result set. She doesn't have friends yet but she shows up in the result set. I want to remove (self) from the results and just display users not yet her friends.

Any ideas? I would gladly appreciate any help. Thanks!

Answer 1

Your Friends table should relate IDs and not Names .

That said, here's the query that will return a list of all non-friends for every user:

SELECT u1.username as `User`, GROUP_CONCAT(u2.username) as `Not friends with`
FROM Users u1
JOIN Users u2 ON u1.id <> u2.id 
LEFT JOIN Friends f ON u1.id = f.user AND u2.id = f.friend   
WHERE f.friend IS NULL
GROUP BY u1.id

SQL Fiddle

The same for a single user ('jim'):

SELECT u1.username as `User`, GROUP_CONCAT(u2.username) as `Not friends with`
FROM Users u1
JOIN Users u2 ON u1.id <> u2.id 
LEFT JOIN Friends f ON u1.id = f.user AND u2.id = f.friend   
WHERE f.friend IS NULL
      AND u1.username = 'jim'

And Single-User Fiddle