Mysql的关联操作（关联查询、关联插入）

Zend Framework中的model中关联查询：

    /**
     * 返回【单个研讨会】的详细信息
     * @param webinarid
     * @return Array
     */
    public function getWebinarInfoById($webinarid) {
        $selectString = 'a.`id` as id,
                                `title`,
                                `begin_time`,
                                `end_time`,
                                `basic_intro`,
                                `detail_intro`,
                                `sponsor_id`,
                                `recommend_if`,
                                `speakername`,
                                `speaker_photo`,
                                `office_position` ,
                                `speakerintro` ,
                                `views` ,
                                `company_name` ,
                                `sponsor_intro` ,
                                `sponsor_logo` ,
                                `url`
                                ';
        $webinarid = intval ( $webinarid );
        $webinarInfo = $this->_db->fetchRow ( "SELECT {$selectString} FROM `cnt_webinar` as a,
                                                                    `cnt_webinar_sponsor` as b
                                                                WHERE a.id={$webinarid} 
                                                                    AND a.sponsor_id = b.id;
                                                                " );
        return $webinarInfo;
    }

关联插入：

有两个表：表A和表B，A表中有id, uid,asker，B表中也有uid,username。现在我想更新A表中的uid变成B表中的uid，条件是A表中的username=B表中的username：

UPDATE  `A`  AS a,`B`  AS b SET a.uid = b.uid WHERE b.username = a.asker