當從連接表中選擇記錄時，如果可能，從B（加入）表中獲取每個A表記錄的最近記錄

Question

我有歷史記錄表並加入了一個備註表。兩者都有日期 - 所以創建了歷史記錄，並且有一個評論表，它記錄了對該歷史記錄的評論。

當我詢問表格時，我需要所有歷史記錄以及每個記錄的最新評論。不幸的是，我似乎沒有按照自己的意願獲取數據，因爲如果將評論添加到較早的歷史記錄中，則此查詢將返回最近的歷史記錄（正確），但會返回最近的整體評論（不正確）而不是我正在看的歷史記錄的最新評論。

這裏是我們的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

試試這個

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

試試這個

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