Question

我有包含id, sender_id, receiver_id, message and conversation_id

的消息的表格

（我通过conversation_id连接他们，他们通过发送第一条消息进行连接，如果有人回复，首先他从他是接收者的消息中搜索conversation_id，并且他回复的人是发送者，并且通过该发送消息具有相同的conversation_id）

现在在消息列表中，我想在每个conversation_id sender_id='$my_id' OR receiver_id='$my_id' DISTINCT输出一个最后一行。SELECT DISTINCT conversation_id, sender_id, message FROM messages WHERE receiver_id='$my_id' ORDER BY id DESC

我正在使用{{1}}，但我总是将所有行作为输出：

{{1}}

Answer 1

试试这个

SELECT id,conversation_id, sender_id, receiver_id, message FROM message WHERE receiver_id='$my_id' GROUP BY `conversation_id` ORDER BY id DESC LIMIT 1

Answer 2

请尝试以下查询：

SELECT 
M.*
FROM messages M
INNER JOIN 
(
    SELECT 
    MAX(id) AS last_id_of_conversation,
    conversation_id
    FROM messages
    GROUP BY conversation_id
) AS t
ON M.id = last_id_of_conversation

<强>解释

SELECT 
 MAX(id) AS last_id_of_conversation,
 conversation_id
FROM messages
GROUP BY conversation_id;

此内部查询将生成一个输出，其中每个conversation_id都会有一行以及对话的最后id值（或最大id）。

稍后在主表（messages）与匹配id上的内部查询返回的结果之间建立内部联接。我希望id是primary key。如果是这样，那么上述查询确保为每个对话带来一行（更具体地说是具有最后一条消息的行）。

修改

SELECT M.* FROM messages M INNER JOIN ( SELECT MAX(id) AS last_id_of_conversation, conversation_id FROM messages WHERE sender_id = ? OR receiver_id = ? GROUP BY conversation_id ) AS t ON M.id = last_id_of_conversation

从每个对话中获取最后一条消息

2 个答案: