SQL query to get my followers except me - mysql

Question

I try to make a sql query to get all my followers except me.
I have the following tables:
users
| id_user | username |
| 1 | xaxa |
| 2 | toto |
| 3 | bubu |
| 4 | yiyi |
| 5 | pepe |
| 6 | sisi |
| 7 | fifi |
| 8 | mama |
| 9 | juju |
| 10 | cece | => me
friends
| id_friend | id_user | id_user_to |
| 1 | 10 | 1 |
| 2 | 2 | 10 |
| 3 | 2 | 1 |
| 4 | 6 | 3 |
| 5 | 2 | 9 |
| 6 | 6 | 7 |
| 7 | 5 | 3 |
| 8 | 10 | 5 |
| 9 | 9 | 8 |
| 10 | 8 | 10 |
I want to have this:
my friends
| id_user | name |
| 1 | xaxa |
| 2 | toto |
| 5 | pepe |
| 8 | mama |
actually I have id_user 10 (me) in the result with this query =>
SELECT id_user, name
FROM `users`
WHERE id_user NOT IN (
SELECT `id_user` FROM `friends`
WHERE ( `id_user` = 10 OR `id_user_to` = 10 ))
OR id_user NOT IN (
SELECT `id_user_to` FROM `friends`
WHERE ( `id_user` = 10 OR `id_user_to` = 10 ))
GROUP BY `id_user`

Answer 1

The solution is a union of two simple joins:
SELECT u.id_user, u.username name
FROM friends f
JOIN users u ON u.id_user = f.id_user_to
WHERE f.id_user = 10
UNION
SELECT u.id_user, u.username
FROM friends f
JOIN users u ON u.id_user = f.id_user
WHERE f.id_user_to = 10
Note that the keyword UNION removes duplicates from the result, so no need to code anything special to handle the case when there's a friend link in both directions between two users (FYI, UNION ALL retains duplicates).
Because at most one index is used per table per query, by splitting the query into two parts, if indexes are created on the user id columns of the friends table, this query will scale well (to millions of users)
There was no need to code anything to handle an "except me" condition, unless you have a row in the friends table for you being your own friend, which you don't.
SqlFiddle: http://sqlfiddle.com/#!9/7cbb3/4

Answer 2

This should do:
SELECT u.id_user, u.username name
FROM `friends` f
JOIN `users` u
ON u.id_user = f.id_user_to and f.id_user = 10
or u.id_user = f.id_user and f.id_user_to = 10
ORDER BY u.id_user
SqlFiddle: http://sqlfiddle.com/#!9/7cbb3/1

Answer 3

So essentially you need to get users who are related in either direction (id_user -> id_user_to OR id_user_to -> id_user)
You can do either one of those with these queries:
SELECT friends.id_user, users.name
FROM users
JOIN friends on users.id_user = friends.id_user
WHERE friends.id_user_to = 10
SELECT friends.id_user_to, users.name
FROM users
JOIN friends on users.id_user = friends.id_user
WHERE friends.id_user = 10
But you want both sides. One way to do it is to do both queries and UNION them together. You could do it like this whilst also adding in the names
SELECT friends.id_user, users.name
FROM users
JOIN friends on users.id_user = friends.id_user
WHERE friends.id_user_to = 10
UNION
SELECT friends.id_user_to, users.name
FROM users
JOIN friends on users.id_user = friends.id_user
WHERE friends.id_user = 10
It's also worth noting that the UNION will only show you distinct rows so if you have users in both directions (for example 1 -> 10 and 10 -> 1) they will not show twice.