Checking field value with submited database

Dear Support,

i would like to ask how to use event switcher to check if the value on some field has been submited before and prevent any duplicate value on that field in database, perhaps something like username / email checking on registration to avoid any double submited data.

i found this forum post, http://www.chronoengine.com/forums/posts/f5/t96748.html
i tried to put everything exactly like on the poster did , but when i tried to submit an submited value i got this error

Notice: Undefined index: input_name in /home/robtan19/nadbet89.com/test/administrator/components/com_chronoforms5/chronoforms/actions/event_switcher/event_switcher.php(26) : eval()'d code on line 11
thanks for registering

the code was :

<?php

$db = JFactory::getDBO();

$query = "

SELECT `id`

FROM `flczt_chronoengine_chronoforms_datatable_Register`

WHERE `norek` = '{$form->data['input_name']}' ;

";

$db->setQuery($query);

$options = $db->loadAssocList();

if ($options) {

echo 'fail';

}else{

return 'success';

}

?>

please guide me , or i'll really appreciate if there is any tutorial for this or for ajax validating will be better since user doesnt need to press submit to know if the value has been used before

thanks
robin

thanks bob you are the live saver, im gonna buy this compt since only this compt have this such a great feature even for someone who are blind with php.