private function _prepareGroupBy(array $groupByArray)
	{
		if(count($groupByArray) === 1)
		{
			foreach($groupByArray as $orderMode => $column)
			{
				/** @var ColumnInterface $column */
				$this->groupBy = 'GROUP BY ' . $column->getColumnName() . ' ';
				if(is_string($orderMode))
				{
					$this->groupBy .= $orderMode . ' ';
				}
			}
		}
	}