CREATE DEFINER=`root`@`localhost` PROCEDURE `make_friend`(
    IN user1 BIGINT unsigned,
    IN user2 BIGINT unsigned
)
BEGIN
    INSERT INTO `friends` (`user1`, `user2`)
    VALUES (user1, user2)
    WHERE NOT EXISTS (
        SELECT * FROM `friends` f
        WHERE f.user1 NOT IN (user1,user2)
        AND f.user2 NOT IN (user1,user2)
    )
END