使用MySQL直接在查询中进行值检查

Question

I've the following table:

friends :

id | user1 | user2   | status
1  | Jorge | Alisson | 3
2  | Lucas | Jorge   | 3

And to do a SELECT , I do (the variable $userLogged refers to the user logged in at the time):

SELECT user1, user2, status
FROM friends
WHERE user1 = '$userLogged' OR user2 = '$userLogged'

So, in PHP:

foreach($query as $q) {
    if($q["user1"] == $userLogged) {
        $userFriend = $q["user2"];
    } else {
        $userFriend = $q["user1"];
    }
}

Suppose the $userLogged is Jorge. I want to get the friend ( $userFriend ), that is, which is not $userLogged (Alisson and Lucas, in case). Currently I do this check in the manner shown above, with PHP. How can I do this directly in the query, thus saving this check in PHP? For ex.: SELECT user1 AND user2 <> '$userLogged' AS userFriend ...

Answer 1

If you have an index on friends(user1, user2, status) and friends(user2, user1, status) , then the fastest way is:

SELECT user2 as `user`, status
FROM friends
WHERE user1 = '$userLogged' 
UNION ALL
SELECT user1, status
FROM friends
WHERE user2 = '$userLogged';

The index can be used for both subqueries, so this should be very fast.

Answer 2

SELECT
CASE WHEN user1 = '$userLogged' THEN user2
ELSE user1 END AS userFriend
FROM friends
WHERE user1 = '$userLogged' OR user2 = '$userLogged'