GROUP BY 分組后得到最新即時間最大的一條數據（需添加limit才可生效）

當使用GROUP BY 分組，默認返回的數據是組中最小的記錄即id最小的數據，

當開發中經常會需要分組后將最新的數據放在前面，

為了實現需求，使用了嵌套查詢，分別使用order by來排序

SELECT
    id,
    from_id,
    to_id,
    conversation_id,
    created_date,
    has_read,
    content
FROM
    ( SELECT * FROM message WHERE from_id = 6 OR to_id = 6 ORDER BY created_date DESC) AS tt 
GROUP BY
    conversation_id 
ORDER BY
    created_date DESC

 

 但執行結果依然是id最小的那條數據 

 

 

最終添上limit max_size 后才生效

SELECT
    id,
    from_id,
    to_id,
    conversation_id,
    created_date,
    has_read,
    content
FROM
    ( SELECT * FROM message WHERE from_id = 6 OR to_id = 6 ORDER BY created_date DESC limit 100) AS tt 
GROUP BY
    conversation_id 
ORDER BY
    created_date DESC