public function addGroupBy($column, $sort = null)
        {
            if (is_array($column)) {
                foreach ($column as $single_column) {
                    $this->addGroupBy($single_column[0], $single_column[1]);
                }
            } else {
                $this->sort_groups[] = new QueryColumnSort($column, $sort);
            }

            return $this;
        }