Unique value in Drupal cck fields

function vs2_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL) {
switch ($op) {
case 'validate':
if ($node->type=='website' ){
$pass='';
$sql = "SELECT field_website_link_url, nid FROM {content_type_website} WHERE field_website_link_url = '".$node->field_website_link[0][url]."' $sub_filter limit 1";
$result = db_query($sql);
$line = db_fetch_array($result);
if ( drupal_strlen($line[field_website_link_url])>0 ) {
$pass = l("Go and check the existing Drupal Website", "node/".$line[nid]);
form_set_error ('field_website_link', "URL already exists [$pass]");
}
}
break;
}
}