[micscr]

Вот еще вариант, уже без подзапроса:

SELECT u.idu, b.idu , b.idb
FROM users u
LEFT JOIN banned b
ON ((b.idu = $idu OR b.idb = $idu) AND (u.idu = b.idu OR u.idu = b.idb))
WHERE ( (b.idu IS NULL) AND (b.idb IS NULL))
ORDER BY RAND()
LIMIT 1