從表中選擇具有不同列值的行

Question

在基於線程的消息傳遞系統中，表架構為

> messages table
id(int auto incr primary key)
body(varchar)
time(datetime)

>message_reference table
id(int auto incr primary key)
message_id(forgain key from message table)
sender
receiver

在這里，我要選擇發送到新的接收者的第一個消息ID，發送者是已登錄的用戶。

用多個查詢和一些代碼執行此操作顯然是可能的，但是可以通過單個查詢來解決性能問題嗎？

Answer 1

你可以試試

編輯：

如果id是自動遞增的，則id也會隨着時間增加，您可以使用：

SELECT message_reference.message_id, message_reference.receiver, messages.body
FROM message_reference, messages
WHERE message_reference.message_id IN (SELECT  MIN(message_reference.message_id)
                            FROM message_reference
                            GROUP BY message_reference.receiver)
AND message_reference.message_id = messages.id AND message_reference.sender = <sender>

Answer 2

這是您想要的最好的猜測，但是如果您提供已知的輸入，示例數據和預期的輸出，則將更加容易。

SELECT
    MR2.message_id
FROM (
    SELECT
        MR.sender,
        MR.receiver,
        M.MIN(`time`) AS min_time
    FROM
        Message_References MR -- Either use plural names (my personal preference) or singular, but don't mix them
    INNER JOIN Messages M ON
        M.id = MR.message_id
    WHERE
        MR.sender = <sender>
    GROUP BY
        MR.received) SQ
INNER JOIN Message_References MR2 ON
    MR2.sender = SQ.sender AND
    MR2.receiver = SQ.receiver AND
    MR2.`time` = SQ.min_time

Answer 3

select mr.message_id from
  message_reference as mr inner join 
    (select mr1.reciever max(m1.time) as time from messages as m1
       inner join message_reference as mr1 on mr1.message_id = m1.id
       group by mr1.reciever) as last
    on mr.reciever = last.reciever and mr.time = last.time

在“接收者和時間”表上使用“每個接收者的最大時間”表加入消息引用

Answer 4

好吧，我得到了答案，只是按查詢分組按我想要的方式工作。 我用查詢

SELECT SENDER,
RECEIVER,
BODY,
TIME,
MESSAGE_ID
FROM MESSAGE_REF JOIN MESSAGE 
ON  MESSAGE.ID=MESSAGE_REF.MESSAGE_ID
ORDER BY 'TIME' GROUP BY RECEIVER`

謝謝大家的幫助。