Различные вопросы по PHP и MySQL

UPDATE `applications` SET `id2` = IF(`id2` = 0, 3, `id2`) WHERE id = 1

SELECT  `id1` ,  `id2` 
FROM applications
WHERE  `id1` =  "3"
OR  `id2` =  "3"
UNION SELECT  `id1` ,  `id2` 
FROM duel
WHERE  `id1` =  "3"
OR  `id2` =  "3"
LIMIT 1

mysql_query("select `applications`.`id1`, `applications`.`id2` from `applications` join `duel` on (`applications`.`id1`=`duel`.`id1` or `applications`.`id2`=`duel`.`id2`) and `applications`.`id1`=$id and `duel`.`id1`=$id limit 1);