UPDATE `table` AS `t`
JOIN (
SELECT MIN(`id`) AS `min`, `value`
FROM `table`
GROUP BY `value`
) AS `t1` ON `t1`.`value` = `t`.`value`
SET `t`.`min_id` = `t1`.`min`$message_part .= "Content-Type: application/xls; name=\"".$realname."\"\n\n";