/**
  * Returns the GROUP BY clauses ready for inserting into a query.
  *
  * @param SQLSelect $query The expression object to build from
  * @param array $parameters Out parameter for the resulting query parameters
  * @return string Completed group part of statement
  */
 public function buildGroupByFragment(SQLSelect $query, array &$parameters)
 {
     $groupBy = $query->getGroupBy();
     if (empty($groupBy)) {
         return '';
     }
     $nl = $this->getSeparator();
     return "{$nl}GROUP BY " . implode(', ', $groupBy);
 }