MySQL:从彼此关联的 2 个表中获取最新消息 [英] MySQL: Get latest message from 2 tables that are associated with eachother

问题描述

首先，非常感谢您的帮助.

我有 2 个表:一个 conversation 表和一个 message 表，以及第三个表 assoc_message__conversation 将消息与对话相关联.

我需要获取为每个指定对话发送的最新 message_id 和 message，以及与之关联的 conversation_id.>

这是一个 db-fiddle:https://www.db-fiddle.com/f/kxRQeGUYYgQ7FTwi96hbLp/0

如您在此示例中所见，有两个 conversation_id 为 1 和 2 的对话，并且每个对话关联了三条消息.消息 1、2 和 3 与会话 1 相关联，消息 4、5 和 6 与会话 2 相关联.

我需要能够在 assoc_message__conversation 表(ID 1 和 2)中指定 conversation_id，并检索最新的 message_id、message 和关联的 conversation_id 从 message 表中为每个指定的对话发送.

所以它应该拉的行是:

conversation_id |message_id |信息------------------------------------------------1 |3 |最新消息"------------------------------------------------2 |6 |最新消息"------------------------------------------------

非常感谢您的帮助！

解决方案

在旧版本的 MySQL (< 8.0.2) 中，我们可以使用 派生表.在派生表中，我们可以获得每个 conversation_id 的最新 send_datetime 值.另外，值得注意的是，您可以在此子查询的WHERE 子句中为conversation_id 提供过滤器.

然后我们可以使用这个子查询的结果集并适当地连接回主表，以获取与对话中最新消息对应的行.

架构(MySQL v5.7)

查看 DB Fiddle

查询 #1

SELECTamc.conversation_id,m.message_id,消息从assoc_message__conversation AS amc加入消息 AS mON m.message_id = amc.message_id加入(选择amc1.conversation_id，MAX(m1.send_datetime) AS latest_send_datetime从assoc_message__conversation AS amc1加入消息 AS m1ON m1.message_id = amc1.message_idWHERE amc1.conversation_id IN (1,2) -- 在这里提供输入过滤器GROUP BY amc1.conversation_id) 作为 dtON dt.conversation_id = amc.conversation_id ANDdt.latest_send_datetime = m.send_datetime;

结果

<代码>|对话_id |message_id |留言 ||--------------- |---------- |-------------- ||1 |3 |最新消息 ||2 |6 |最新消息 |

<小时>

在 MySQL 8.0.2 及更高版本中，我们可以使用 Row_Number() 功能.在 conversation_id 的一个分区内，我们将确定每条消息的 Row Number，按照 send_datetime 的降序排序.在此子查询中，您可以在 WHERE 子句中为 session_id 提供过滤器.

然后我们将使用这个结果集作为派生表，并只考虑那些行号值为 1 的行(因为它将属于最新的 send_datetime).

架构(MySQL v8.0)

查看 DB Fiddle

查询 #2

SELECTdt.conversation_id,dt.message_id,dt.message从(选择amc.conversation_id,m.message_id,m. 消息，ROW_NUMBER() OVER(PARTITION BY amc.conversation_idORDER BY m.send_datetime DESC) AS row_no从assoc_message__conversation AS amc加入消息 AS mON m.message_id = amc.message_idWHERE amc.conversation_id IN (1,2) -- 在这里提供输入过滤器) 作为 dt哪里 dt.row_no = 1;

结果

<代码>|对话_id |message_id |留言 ||--------------- |---------- |-------------- ||1 |3 |最新消息 ||2 |6 |最新消息 |

First, thank you so much for your help.

I have 2 tables: a conversation table and a message table, and a third table assoc_message__conversation that associates the messages to a conversation.

I need to get the latest message_id and message sent for each conversation specified, along with the conversation_id it is associated with.

Here is a db-fiddle: https://www.db-fiddle.com/f/kxRQeGUYYgQ7FTwi96hbLp/0

As you can see in this example, there are two conversations with conversation_id of 1 and 2, and there are three messages associated to each conversation. Messages 1, 2, and 3 are associated to conversation 1, and messages 4, 5 and 6 are associated to conversation 2.

I need to be able to specify the conversation_id's in the assoc_message__conversation table (IDs 1 and 2), and retrieve the latest message_id, message and the associated conversation_id sent from the message table for each conversation specified.

So the rows it should pull are:

conversation_id | message_id | message
------------------------------------------------
              1 |          3 | "Latest message"
------------------------------------------------
              2 |          6 | "Latest message"
------------------------------------------------

Thank you so much for your help!

解决方案

In older versions of MySQL (< 8.0.2), we can use Derived Tables. In a Derived table, we can get the latest send_datetime value for each conversation_id. Also, it is noteworthy that you can provide your filters for conversation_id in the WHERE clause of this subquery.

We can then use this subquery`s result-set and join back to the main tables appropriately, to get the row corresponding to the latest message in a conversation.

Schema (MySQL v5.7)

View on DB Fiddle

Query #1

SELECT
  amc.conversation_id, 
  m.message_id, 
  m.message   
FROM 
  assoc_message__conversation AS amc
JOIN message AS m 
  ON m.message_id = amc.message_id 
JOIN 
(
  SELECT
    amc1.conversation_id, 
    MAX(m1.send_datetime) AS latest_send_datetime
  FROM
   assoc_message__conversation AS amc1
  JOIN message AS m1
    ON m1.message_id = amc1.message_id 
  WHERE amc1.conversation_id IN (1,2)  -- Here you provide your input filters
  GROUP BY amc1.conversation_id
) AS dt  
  ON dt.conversation_id = amc.conversation_id AND 
     dt.latest_send_datetime = m.send_datetime;

Result

| conversation_id | message_id | message        |
| --------------- | ---------- | -------------- |
| 1               | 3          | Latest message |
| 2               | 6          | Latest message |

---

In MySQL 8.0.2 and above, we can use Row_Number() functionality. Within a partition of conversation_id, we will determine Row Number for every message, sorted in descending order of send_datetime. In this subquery, you can provide your filters for conversation_id in the WHERE clause.

We will then use this result-set as a Derived Table, and consider only those rows, where Row Number value is 1 (as it will belong to latest send_datetime).

Schema (MySQL v8.0)

View on DB Fiddle

Query #2

SELECT 
  dt.conversation_id, 
  dt.message_id, 
  dt.message 
FROM 
(
  SELECT
    amc.conversation_id, 
    m.message_id, 
    m.message, 
    ROW_NUMBER() OVER (PARTITION BY amc.conversation_id 
                       ORDER BY m.send_datetime DESC) AS row_no 
  FROM
   assoc_message__conversation AS amc
  JOIN message AS m
    ON m.message_id = amc.message_id 
  WHERE amc.conversation_id IN (1,2)  -- Here you provide your input filters
) AS dt  
WHERE dt.row_no = 1;

Result

| conversation_id | message_id | message        |
| --------------- | ---------- | -------------- |
| 1               | 3          | Latest message |
| 2               | 6          | Latest message |

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