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);
}
}