public function columnKeys()
    {
        $columnKeys = [];

        foreach ($this->columns() as $key => $column) {
            if (is_string($key)) {
                $columnKeys[] = $key;
            } elseif (str_contains($column, '.')) {
                list($table, $columnKey) = explode('.', $column);
                $columnKeys[]            = $columnKey;
            } else {
                $columnKeys[] = $column;
            }
        }

        return $columnKeys;
    }