private function querySingleValue($sql, $parameters = [])
    {
        try {
            $statement = $this->pdo->prepare($sql);
            $statement->execute($parameters);

            $value = $statement->fetchColumn();

            $statement->closeCursor();
            if ($value === false) {
                throw new StorageException("The query returned no result.");
            }
            return $value;
        } catch (\PDOException $e) {
            throw new StorageException("The query failed.", 0, $e);
        }
    }