MySQL获取每个对话的最后一条消息

Question

我对 mysql 有很大的问题，因为我是前端开发人员，而且我对 SQL 很差 ^^

我在数据库 + 用户表中有 3 个表

参与者：

+----+---------+-----------------+---------------------+
| id | user_id | conversation_id |          updated_at |
+----+---------+-----------------+---------------------+
|  1 |       1 |               1 | 2022-01-03 00:00:00 |
|  2 |       2 |               1 | 2022-01-03 00:00:00 |
|  3 |       1 |               2 | 2022-01-03 00:00:00 |
|  4 |       3 |               2 | 2022-01-03 00:00:00 |
|  5 |       1 |               2 | 2022-01-03 00:00:00 |
+----+---------+-----------------+---------------------+

对话：

+----+---------------------+
| id |          updated_at |
+----+---------------------+
|  1 | 2022-01-03 00:00:00 |
|  2 | 2022-01-03 00:00:00 |
+----+---------------------+

留言：

+----+----------------+-----------------+-------------------+----------------------+
| id | participant_id | conversation_id |           message |           updated_at |
+----+----------------+-----------------+-------------------+----------------------+
|  1 |              1 |               1 | test msg 1 room 1 |  2022-01-03 00:00:00 |
|  1 |              1 |               1 | test msg 2 room 1 |  2022-01-05 00:00:00 |
|  1 |              2 |               1 | test msg 3 room 1 |  2022-01-07 00:00:00 |
|  1 |              3 |               2 | test msg 1 room 2 |  2022-01-03 00:00:00 |
|  1 |              2 |               2 | test msg 2 room 2 |  2022-01-04 00:00:00 |
|  1 |              1 |               2 | test msg 3 room 2 |  2022-01-05 00:00:00 |
|  1 |              1 |               1 | test msg 4 room 1 |  2022-01-14 00:00:00 |
+----+----------------+-----------------+-------------------+----------------------+

所以我试图从每个房间获取最后一条消息，但我有问题，当我尝试分组时，我有这样的错误

错误代码：1055。SELECT 列表的表达式 #1 不在 GROUP BY 子句中，并且包含在功能上不依赖于 GROUP BY 子句中的列的非聚合列“zzzz”； 这与 sql_mode=only_full_group_by 不兼容

有可能得到每个房间的最后一条消息吗？ 像这样

+-----------------+-------------------+----------------------+
| conversation_id |           message |           updated_at |
+-----------------+-------------------+----------------------+
|               2 | test msg 3 room 2 |  2022-01-05 00:00:00 |
|               1 | test msg 4 room 1 |  2022-01-14 00:00:00 |

我尝试过这样的事情，但没有工作......

WITH RECURSIVE test AS (
    SELECT
        pp.user_id,
        pp.conversation_id,
        pp.updated_at
    FROM
        participants pp
    UNION
        SELECT
            p.user_id,
            p.conversation_id,
            p.updated_at
        FROM
            participants p
        INNER JOIN conversations c ON c.id = p.conversation_id
        
) SELECT
    m.conversation_id,
    m.message,
    m.updated_at
FROM
    test t
    
INNER JOIN messages as m ON  m.conversation_id = t.conversation_id 
ORDER BY m.updated_at DESC
;

Answer 1

如果我正确理解你的问题。 您需要的一切都可以来自消息表。 所以你可以尝试这样的事情：

WITH cte as(
SELECT
       RIGHT (message,LENGTH(message)-LOCATE('r',message)) AS room,
       MAX(LEFT (message,LENGTH(message)-LOCATE('m',message))) AS msg,
       MAX(updated_at) AS updated_at
FROM messages
group by room
)

select * 
FROM messages
WHERE RIGHT (message,LENGTH(message)-LOCATE('r',message)) in (select room from cte)
AND   LEFT (message,LENGTH(message)-LOCATE('m',message)) in (select msg from cte)
AND   updated_at in (select updated_at from cte)

分贝小提琴

如果我误解了什么，请告诉我。