Question

你好我有这个SQL女巫应该给我max(messages.message_id) 所以我记录的第一个message_id是7，最后一个message_id是10，但不是给我10它给了我7 ...它完全忽略了MAX(messages.message_id)和给我第一个message_id ...关于如何解决此问题的任何建议？

SELECT
  profile.first_name,
  profile.last_name,
  conversations.conversation_hash,
  conversations.person_a,
  conversations.person_b,
  messages.conversation_hash,
  MAX(messages.message_id),
  messages.message,
  messages.subject,
  messages.date
FROM conversations
  INNER JOIN messages
    ON conversations.conversation_hash = messages.conversation_hash
  INNER JOIN profile
    ON profile.id = conversations.person_b
WHERE conversations.person_a = '$id'
GROUP BY messages.conversation_hash
ORDER BY messages.message_id DESC

表：会话：

conversation_id | conversation_hash | person_a | person_b |

消息：

conversation_hash | from_id | to_id | message_id | subject | message | date

Answer 1

尝试这样的事情：

SELECT
    profile.first_name,
    profile.last_name,
    conversations.conversation_hash,
    conversations.person_a,
    conversations.person_b,
    messages.conversation_hash, 
    messages.message_id,
    messages.message, 
    messages.subject, 
    messages.date
FROM conversations
INNER JOIN (SELECT MAX(message_id) as maxMessageId, conversation_hash 
            FROM messages
            GROUP BY conversation_hash) m 
       ON conversations.conversation_hash=m.conversation_hash
    INNER JOIN messages
        ON conversations.conversation_hash=messages.conversation_hash AND message.message_id = m.maxMessageId
    INNER JOIN profile
       ON profile.id=conversations.person_b
    WHERE conversations.person_a='$id'

祝你好运。

Answer 2

您可以在子查询中单独从表message_id获取最新的messages，然后将其结果与表联接，前提是它在两个条件下匹配：conversation_hash和message_id。

完整查询：

SELECT  profile.first_name,
        profile.last_name,
        conversations.conversation_hash,
        conversations.person_a,
        conversations.person_b,
        messages.*
FROM    conversations
        INNER JOIN messages
            ON conversations.conversation_hash = messages.conversation_hash
        INNER JOIN  
        (
            SELECT  conversation_hash, MAX(message_id) max_ID
            FROM    messages
            GROUP   BY conversation_hash
        ) c ON messages.conversation_hash = c.conversation_hash AND
                messages.message_id = c.max_ID
        INNER JOIN profile
            ON profile.id=conversations.person_b
WHERE   conversations.person_a='$id'
ORDER   BY messages.message_id DESC

MAX（）SQL不起作用

2 个答案: