CRM-13838 - Report filtering by price level

http://issues.civicrm.org/jira/browse/CRM-13838

CRM/Report/Form/Event/ParticipantListing.php

@@ -552,6 +552,10 @@ GROUP BY  cv.label
     }
   }
 
+  function groupBy(){
+    $this->_groupBy = "GROUP BY {$this->_aliases['civicrm_participant']}.id";
+  }
+
   function postProcess() {
 
     // get ready with post process params