protected function buildGroupBy() {
		if(!$this->groupBy || !$this->groupBy[0])
			return ['', []];

		$groupBy = $this->groupBy;
		$groupBySql = $groupBy[0];
		$groupByParameters = $groupBy[1];

		$res = [];

		foreach(explode(',', $groupBySql) as $column) {
			if($this->isIdentifier(trim($column)))
				$res[] = $this->replace(trim($column));
			else
				$res[] = trim($column);
		}

		$sql = "\n".'GROUP BY '.implode(', ', $res);

		return [$sql, $groupByParameters];
	}