/**
  * @param string $alias optional label for the result of the aggregator
  */
 function __construct($alias = null)
 {
     parent::__construct(null, $alias);
 }
 /**
  * @return SQLFunction
  **/
 protected function getFunction(Criteria $criteria, JoinCapableQuery $query)
 {
     return parent::getFunction($criteria, $query)->setAggregateDistinct();
 }