public function setDefaultNull($fieldNames)
    {
        if (is_array($fieldNames))
        {
            foreach ($fieldNames as $field_name)
            {
                $this->setDefaultNull($field_name);
            }
        }
        else
        {
            if (isset($this->m_fields[$fieldNames]))
            {
                $this->m_fields[$fieldNames]->setDefault('NULL');
            }
            else
            {
                $err_msg = '[' . $fieldNames . '] field not found in table ' .
                           '[' . $this->m_name . '] when setting null fields';

                throw new \Exception($err_msg);
            }
        }
    }