Join the banners table.

$query->select($db->quoteName(array('a.id','a.name','contact','email','extrainfo','state','checked_out','checked_out_time','metakey','own_prefix','metakey_prefix','purchase_type','track_clicks','track_impressions')));
$query->from($db->quoteName('#__banner_clients a'))
  ->join('inner', '#__banners b ON b.client_id = a.id');

Join the banners table.

$query->select($db->quoteName(array('a.id','a.name','contact','email','extrainfo','state','checked_out','checked_out_time','metakey','own_prefix','metakey_prefix','purchase_type','track_clicks','track_impressions')));
$query->from('#__banner_clients a')
  ->join('inner', '#__banners b ON b.client_id = a.id');