SQL Friends List Query

Question

I am new to this database SQL language so I will try to make it as simple as possible. (I am using MySQL Workbench)

I have a table for User and I want multiple users to

Answer 1

You can use INto check if either column is equal to the ID of the user you want to look up and a CASE ... END to take the column which is not equal to the ID of the user you want to look up.

SELECT CASE
         WHEN user_id = 2
           THEN user_friend_id
         WHEN user_friend_id = 2
           THEN user_id
       END friend
       FROM friends
       WHERE 2 IN (user_id, user_friend_id);

Alternatively you could use a UNION ALL approach, which might perform better as it can use indexes on user_id or user_friend_id.

SELECT user_id friend
       FROM friends
       WHERE user_friend_id = 2
UNION ALL
SELECT user_friend_id friend
       FROM friends
       WHERE friend_id = 2;

But this is only better if there are such indexes. If there aren't, it will need two scans on the table opposed to the first approach only needing one. So it's worse in that case.

Answer 2

Use UNION ALL to get all friends, parameterize this query:

select friend_user_id as friend_id from friends f where f.user_id = 2 --parameter
union all
select user_id as friend_id from friends f where f.friend_user_id = 2 --parameter