public function update($set = array())
    {
        if (is_array($set)) {
            $params = array();
            foreach ($set as $field => $param) {
                $this->add('set', $field . ' = ?', true);
                $params[] = $param;
            }
            $this->params = array_merge($params, $this->params);
        } else {
            $this->add('set', $set, true);
        }
        $this->type = self::UPDATE;
        return $this->execute();
    }