public function getColumnsAsSQL($tableAlias = '', $columnPrefix = '')
    {
        if (empty($this->mapping)) {
            return '*';
        }

        $output = '';
        foreach ($this->mapping as $column => $mappingField) {
            if (!empty($output)) {
                $output .= ',';
            }

            if (!empty($tableAlias)) {
                $output .= '`' . $tableAlias . '`.';
            }

            $output .= '`' . $column . '`';

            if (!empty($columnPrefix)) {
                $output .= ' as `' . $columnPrefix . $column . '`';
            }
        }

        return $output;
    }