Change Value of a Field Element on Change in Dropdown

[sanyamjain]

Hi, I am making a tool where on selecting a drop down, I want to update the value of another element.

On selection a Service from the dropdown list, I want to load the rate of the service on a field element.

Invoice Table:
ozone_invoice___Service1
ozone_invoice___Service1_Rate

Service Table:
ozone_services___id
ozone_services___Service_Rate

I have placed this code on Javascriopt action change

$db =&JFactory::getDBO();
$db->setQuery('SELECT `Service_Rate` FROM `ozone_services` where `id` = '2'');
$fieldA = $db->loadResult();
return $fieldA;

$('ozone_invoice___Service1_Rate').value = $fieldA;

But this is not working, please help.