当从连接表中选择记录时，如果可能，从B（加入）表中获取每个A表记录的最近记录

Question

我有历史记录表并加入了一个备注表。两者都有日期 - 所以创建了历史记录，并且有一个评论表，它记录了对该历史记录的评论。

当我询问表格时，我需要所有历史记录以及每个记录的最新评论。不幸的是，我似乎没有按照自己的意愿获取数据，因为如果将评论添加到较早的历史记录中，则此查询将返回最近的历史记录（正确），但会返回最近的整体评论（不正确）而不是我正在看的历史记录的最新评论。

这里是我们的MySQL

SELECT h.id 
    , c.id comment_id 
    , c.comment recent_comment 
    , h.* 
    FROM crm_device_history h 
    LEFT 
    JOIN crm_device_history_comments c 
    ON c.crm_device_history_id = h.id 
    AND c.id = (SELECT max(id) 
        FROM crm_device_history_comments 
       WHERE c.crm_device_history_id = h.id) 
WHERE device_id = 147 
    AND crm_history_states_id >= 0 
ORDER 
    BY h.id DESC 

crm_device_history表

id 
device_id 
crm_history_states_id 
userID 
dateTime 
system_comment 
comment -> this field is to be dropped now we have a separate table 
distributor_assignment 
client_assignment 
updated_date 
created_date 

crm_device_history_comments

id 
crm_device_history_id 
comment 
user_id 
updated_date 

Answer 1

试试这个

SELECT h.id, 
     c.id  AS comment_id, 
     c.comment AS recent_comment, 
     h.* 
FROM crm_device_history h 
     LEFT JOIN (SELECT Max([date]), 
         id 
        FROM crm_device_history_comments 
        GROUP BY id) c 
       ON c.crm_device_history_id = h.id 
WHERE device_id = 147 
     AND crm_history_states_id >= 0 
ORDER BY h.id DESC 

Answer 2

试试这个

SELECT 
     h.id, 
     sub_query.id AS comment_id, 
     c.comment as recent_comment, 
     h.* 
    FROM 
     crm_device_history h 
    INNER JOIN 
     (SELECT 
      max(id) as id, 
      crm_device_history_id 
     FROM 
      crm_device_history_comments 
     GROUP BY 
      crm_device_history_id) 
     AS sub_query ON sub_query.crm_device_history_id = h.id 
    INNER JOIN 
     crm_device_history_comments AS c ON c.id = sub_query.id 
    WHERE device_id = 147 AND h.crm_history_states_id >= 0 
    ORDER BY h.id DESC