SELECT nick
FROM usuario_eol
WHERE user_id in (
SELECT user_id
FROM
(SELECT distinct user_id
FROM poll
WHERE poll_id ='xxxxx'
ORDER BY dbms_random.value)
WHERE rownum = 1
);
Incluso en este hipotético caso se podría directamente eliminar a los spammers/trampas de la lista por apañaos :
SELECT nick
FROM usuario_eol
WHERE user_id in
(
SELECT user_id
FROM
(SELECT user_id
FROM poll
WHERE user_id in
(SELECT user_id
FROM poll
GROUP BY user_id
HAVING count(user_id)=1)
and poll_id='xxxxx'
ORDER BY dbms_random.value)
WHERE rownum = 1
);