public function rollBack() {
		if ($this->_transactionDepth == 0) {
			throw new PDOException('Rollback error : There is no transaction started');
		}

		$this->_transactionDepth--;

		if ($this->_transactionDepth == 0) {
			parent::rollBack();
		} else {
			$this->exec("ROLLBACK TO SAVEPOINT LEVEL{$this->_transactionDepth}");
		}
	}