Select the users are not friend of a specific user - mysql

Question

Hi, I have these two tables: users and friends (friend_status = 1 means the request is sent, friend_status = 2 means they are friends). Now I want to select all users are not friend of a specific user. How to do?
Assuming the current user is 1. I tried this SQL. It works but it's too long and slow. The first selects all users sent request to user1 but not accepted. The second selects all users receive request from user1. The third and the fourth selects all users is not in "friends" table.
SELECT user_id, name, email
FROM
(
SELECT user_id, name, email
FROM users u INNER JOIN friends f ON u.user_id = f.sender
WHERE f.receiver = 1 AND friend_status <> 2
UNION
SELECT user_id, name, email
FROM users u INNER JOIN friends f ON u.user_id = f.receiver
WHERE f.sender = 1 AND friend_status <> 2
UNION
SELECT u.user_id, u.name, u.email
FROM users u LEFT JOIN friends f ON u.user_id = f.sender
WHERE f.receiver IS NULL
GROUP BY user_id
UNION
SELECT u.user_id, u.name, u.email
FROM users u LEFT JOIN friends f ON u.user_id = f.receiver
WHERE f.sender IS NULL
GROUP BY user_id
) T
GROUP BY user_id
Update: Add a pic.

Answer 1

SELECT
a.user_id,
a.name,
a.email,
b.status IS NOT NULL AS friend_status
FROM
users a
LEFT JOIN
friends b ON
a.user_id IN (b.sender, b.receiver) AND
1 IN (b.sender, b.receiver)
WHERE
(b.friend_id IS NULL OR b.status <> 2) AND
a.user_id <> 1
You had asked a question previously here - "Select users who aren't friends with anyone", and I provided an answer which utilized a LEFT JOIN.
Building off of that, to select users who aren't friends with a specific user, we just simply need to add that specific user's ID to the LEFT JOIN condition (1 IN (b.sender, b.receiver).
Minor Edit: Unless the user can friend him/herself, it wouldn't make sense to also select the user who we're selecting against!! So I added WHERE a.user_id <> 1.

Answer 2

Assuming you want to perform the query on user_id 1:
SELECT user_id, name, email
FROM users AS u
WHERE NOT EXISTS (SELECT * FROM friends AS f
WHERE (f.sender = u.user_id AND f.receiver = 1 AND f.friend_status = 2)
OR (f.sender = 1 AND f.receiver = u.user_id AND f.friend_status = 2)
)
AND u.user_id <> 1
The subquery fetches all the established friendship relationship in which user 1 is either the sender or the receiver. The outer query selects all users for which no such relationship exists. The user with ID 1 is excluded from the query using the last line, as, even if he cannot be friend with himself, I suppose that he should not appear in the final query result.

Answer 3

You may be able to simplify this by using something like this:
SELECT user_id, name, email
FROM
(
SELECT u.user_id, u.name, u.email
FROM users u LEFT JOIN friends f ON u.user_id = f.sender
WHERE IFNULL(friend_status,0) <> 2
GROUP BY user_id
UNION
SELECT u.user_id, u.name, u.email
FROM users u LEFT JOIN friends f ON u.user_id = f.receiver
WHERE IFNULL(friend_status,0) <> 2
GROUP BY user_id
) T
GROUP BY user_id
The IFNULL function returns the value of the first parameter, replacing NULLs with the value of the value second parameter. In this case it means that friend_status will be treated as 0 if there is no matching friend in the friends table, which allows you to reduce the number of selects in the UNION by half.

Answer 4

Try this query
select
u.user_id,
u.name,
u.email,
ifnull(f.friend_status,0) as Relation
from users as u
left join friends as f
on f.sender = u.user_id
where u.user_id not in(select
sender
from friends
where sender = 1)
Here sender = 1 means the user id = 1. You can pass user id to restrict this condition. Also status 0 means he is not friend. and 1 , 2 , 3 are according to your rules