public function makeSelect(array $columns = null)
    {
        $columns = $columns ?: $this->columns();
        $selects = [];

        foreach ($columns as $key => $select) {
            if (is_int($key)) {
                $selects[] = $select;
            } else {
                $selects[] = DB::raw($select.' as '.$key);
            }
        }

        return $selects;
    }