Pass query String to Page query

Hi
I am trying to pass a query string called postcode (your zip code in the states) from one page to another. On the 2nd page I want to use postcode to filter some details of people to identify the person with the relevant postcode.

My query string for example is –

/registration/?postcode = 2880

I know that on the following line of code –
<strong>where tblcoordinator_id_contact_details.Postcode = postcode");</strong> which is about line 25 on page 2
if I put in the actual value of the postcode i.e.. ‘2880’ it gives me the correct result.
if I put in the querystring name of postcode i.e.. ‘postcode’ it gives me every person.
if I put in the function name of the postcode i.e.. $vars it gives me no results.

So I am at a real loss simple process. But not wanting to do posts using a page and filtering. I need help desperately. Please..

So the code that I am using on page 2 or the registration page is –
(by the way I am using a short codeplugin – Shortcode exec PHP)

global $wpdb;
function add_query_vars_filter( $vars ){
  $vars[] = "postcode";
  return $vars;
}
add_filter( 'query_vars', 'add_query_vars_filter' );
$result = $wpdb->get_results( "SELECT tblcoordinator_id_contact_details.ID,
tblcoordinator_id_contact_details.Firstname,
tblcoordinator_id_contact_details.LastName,
tblcoordinator_id_contact_details.Fullname,
tblcoordinator_id_contact_details.Location,
tblcoordinator_id_contact_details.Street,
tblcoordinator_id_contact_details.Postal,
tblcoordinator_id_contact_details.Town,
tblcoordinator_id_contact_details.State,
tblcoordinator_id_contact_details.Postcode,
tblcoordinator_id_contact_details.Phone1,
tblcoordinator_id_contact_details.Phone2,
tblcoordinator_id_contact_details.Fax,
tblcoordinator_id_contact_details.Email
FROM tblcoordinator_id_contact_details
where tblcoordinator_id_contact_details.Postcode = postcode");
//echo "
<pre>"; print_r($result); echo "</pre>
";
echo "Firstname"."  "."LastName"."

";
foreach($result as $row)
 {
 echo $row->Firstname."  ".$row->LastName."";
 }

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Thanks
Peter