SELECT * FROM (
SELECT `id`, 1 AS `fieldNum`, `apropid1` AS `apropid` FROM `table`
UNION SELECT `id`, 2 AS `fieldNum`, `apropid2` AS `apropid` FROM `table`
UNION SELECT `id`, 3 AS `fieldNum`, `apropid3` AS `apropid` FROM `table`
...
) AS `t`
WHERE `t`.`apropid` == 'something'