同時使用時,先group by再orede by,
所以要獲取最新的資訊,就要先max(time)再group by,

例子

SELECT
    `content`,
    `dialogue_content`.`fuserid` AS `fuserid`,
    `read_new`,
    `add_time`,
    `count`,
    `chat_type` 
FROM
    ( SELECT `content`, `fuserid`, `add_time`, `chat_type` FROM `shopwwi_dialogue` WHERE `touserid`
 
 = '10021'  ) `dialogue_content`
    RIGHT JOIN (
SELECT
    sum( IF ( isread = 0, 1, 0 ) ) AS `count`,
    `fuserid`,
IF
    ( min( isread ) = 0, 0, 1 ) AS `read_new`,
    max( add_time ) AS `max_time` 
FROM
    `shopwwi_dialogue` 
WHERE
    `touserid` = '10021' 
GROUP BY
    fuserid 
    ) dialogue_read ON
 
 `dialogue_content`.`fuserid` = `dialogue_read`.`fuserid` 
    AND add_time = max_time 
GROUP BY
    dialogue_content.fuserid 
ORDER BY
    add_time DESC