Mysql 表在使用join时获取多行

我想从3个表pm_conv、user、photo中获取数据，但在加入第3个表photo后，我得到了多行数据，我尝试分配以获取限制为1的数据，但无法

这是一个问题

试试这个

SELECT 
    pm_conv. *, user.username, user.id, photo.url 
FROM 
    pm_conv 
JOIN 
    user ON CASE 
        WHEN pm_conv.sender_id ='2869' 
            THEN pm_conv.recipient_id = user.id 
        WHEN pm_conv.recipient_id ='2869' 
            THEN pm_conv.sender_id = user.id 
        END
JOIN 
    photo ON CASE 
        WHEN pm_conv.sender_id ='2869' 
            THEN pm_conv.recipient_id = photo.user_id 
        WHEN pm_conv.recipient_id ='2869' 
            THEN pm_conv.sender_id = photo.user_id 
        END
WHERE 
    `sender_id`='2869' 
    OR `recipient_id` ='2869' 
ORDER BY  
    `last_answer_date` DESC limit 1

修改的查询：-

---

我想它会提供您想要的输出。

请尝试子查询，以获得照片表。也许它应该起作用

SELECT 
        pm_conv. *, 
        user.username, 
        user.id, 
        IF(pm_conv.sender_id ='2869', 
            (SELECT photo.user_id FROM photo WHERE pm_conv.recipient_id = photo.user_id LIMIT 1 ) , 
            IF (pm_conv.recipient_id ='2869', 
                (SELECT photo.user_id FROM photo WHERE pm_conv.recipient_id = photo.user_id LIMIT 1 ), 
                '')) as PHOTO_USER
    FROM 
        pm_conv 
    JOIN 
        user ON CASE 
            WHEN pm_conv.sender_id ='2869' 
                THEN pm_conv.recipient_id = user.id 
            WHEN pm_conv.recipient_id ='2869' 
                THEN pm_conv.sender_id = user.id 
            END

    WHERE 
        `sender_id`='2869' 
        OR `recipient_id` ='2869' 
    ORDER BY  
        `last_answer_date` DESC

---

你有什么错误吗？你想实现什么？考虑到这是一个对话，并且您的查询意味着您希望获取涉及用户2869的所有消息，您将得到多个结果。我不明白的是，你说即使你使用了极限1，你仍然会得到多个结果？你可能把你的限额放错了1。是的，这是一张基于对话的3号桌。我想分别从这些表中获取conv、username和photo，但photo表给出了多行，因为它有多张照片与一个id，用户当前使用的照片的条件是什么？photo表中是否有其他相关列可以解决此问题？我正在将pm_conv表中的id与photo的user_id列进行匹配，例如id=3562，并且在photo.user_id中有4行与user_id列。感谢您的回答。但是它限制了整个数据我只想限制photo.url好的，你的photo.url在表中有重复的值。是的…因为它有更多的图像urlid@ArslanAli请尝试修改后的查询，它会对您有所帮助。非常感谢您先生：您救了我一天：很高兴为您效劳

SELECT 
    pm_conv. *, user.username, user.id, photo.url 
FROM 
    pm_conv 
JOIN 
    user ON CASE 
        WHEN pm_conv.sender_id ='2869' 
            THEN pm_conv.recipient_id = user.id 
        WHEN pm_conv.recipient_id ='2869' 
            THEN pm_conv.sender_id = user.id 
        END
JOIN ((SELECT `photo`.* FROM `photo` join `pm_conv` as pmc ON CASE 
        WHEN pmc.sender_id ='2869' 
            THEN pmc.recipient_id = photo.user_id 
        WHEN pmc.recipient_id ='2869' 
            THEN pmc.sender_id = photo.user_id 
        END limit 1)) as photo
     ON CASE 
        WHEN pm_conv.sender_id ='2869' 
            THEN pm_conv.recipient_id = photo.user_id 
        WHEN pm_conv.recipient_id ='2869' 
            THEN pm_conv.sender_id = photo.user_id 
        END
WHERE 
    `sender_id`='2869' 
    OR `recipient_id` ='2869' 
ORDER BY  
    `last_answer_date` DESC

SELECT 
        pm_conv. *, 
        user.username, 
        user.id, 
        IF(pm_conv.sender_id ='2869', 
            (SELECT photo.user_id FROM photo WHERE pm_conv.recipient_id = photo.user_id LIMIT 1 ) , 
            IF (pm_conv.recipient_id ='2869', 
                (SELECT photo.user_id FROM photo WHERE pm_conv.recipient_id = photo.user_id LIMIT 1 ), 
                '')) as PHOTO_USER
    FROM 
        pm_conv 
    JOIN 
        user ON CASE 
            WHEN pm_conv.sender_id ='2869' 
                THEN pm_conv.recipient_id = user.id 
            WHEN pm_conv.recipient_id ='2869' 
                THEN pm_conv.sender_id = user.id 
            END

    WHERE 
        `sender_id`='2869' 
        OR `recipient_id` ='2869' 
    ORDER BY  
        `last_answer_date` DESC