public function _render_statements()
    {
        $result = [];

        if (isset($this->args['dropField'])) {
            foreach ($this->args['dropField'] as $field => $junk) {
                $result[] = 'drop column '.$this->_escape($field);
            }
        }

        if (isset($this->args['newField'])) {
            foreach ($this->args['newField'] as $field => $option) {
                $result[] = 'add column '.$this->_render_one_field($field, $option);
            }
        }

        if (isset($this->args['alterField'])) {
            foreach ($this->args['alterField'] as $field => $option) {
                $result[] = 'change column '.$this->_escape($field).' '.$this->_render_one_field($field, $option);
            }
        }

        return implode(', ', $result);
    }