protected function get_component_select_sql($alias = '') {
        $component = $this->get_component();
        if ($alias) {
            $alias = $alias.'.';
        }
        if (empty($component)) {
            $componentwhere = "{$alias}component IS NULL";
            $component = null;
        } else {
            $componentwhere = "({$alias}component IS NULL OR {$alias}component = :component)";
        }
        return array($componentwhere, $component);
    }