public function toSQL(Parameters $params, bool $inner_clause)
    {
        $drv = $params->getDriver();
        $clauses = $this->getClauses();

        $strs = array();
        foreach ($clauses as $clause)
            $strs[] = $drv->toSQL($params, $clause);

        if (count($strs) === 0)
            return;

        return "ORDER BY " . implode(", ", $strs);
    }