選擇具有按兩列分組的最大值的行

Question

我已經看到了很多關於這類問題的解決方案（特別是這一個SQL只選擇列上具有最大值的行 ），但這些似乎都不合適：

我有以下表格布局，附件的版本控制，綁定到實體：

TABLE attachments
+------+--------------+----------+----------------+---------------+
| id   | entitiy_id   | group_id | version_number | filename      |
+------+--------------+----------+----------------+---------------+
| 1    | 1            | 1        | 1              | file1-1.pdf   |
| 2    | 1            | 1        | 2              | file1-2.pdf   |
| 3    | 1            | 2        | 1              | file2-1.pdf   |
| 4    | 2            | 1        | 1              | file1-1.pdf   |
| 5    | 2            | 1        | 2              | file1-2.pdf   |
| 6    | 2            | 3        | 1              | file3-1.pdf   |
+------+--------------+----------+----------------+---------------+

輸出應該是最大版本號，按group_id和entity_id分組，如果有幫助，我只需要單個entity_ids的列表：

+------+--------------+----------+----------------+---------------+
| id   | entitiy_id   | group_id | version_number | filename      |
+------+--------------+----------+----------------+---------------+
| 2    | 1            | 1        | 2              | file1-2.pdf   |
| 3    | 1            | 2        | 1              | file2-1.pdf   |
| 5    | 2            | 1        | 2              | file1-2.pdf   |
| 6    | 2            | 3        | 1              | file3-1.pdf   |
+------+--------------+----------+----------------+---------------+

我想出的是這個自我加入：

SELECT *
FROM   `attachments` `attachments`
       LEFT OUTER JOIN attachments t2
         ON ( attachments.group_id = t2.group_id
              AND attachments.version_number < t2.version_number )
WHERE  ( t2.group_id IS NULL )
   AND ( `t2`.`id` = 1 )
GROUP  BY t2.group_id

但是這個只有在不同的實體不共享相同的組號時才有效。 不幸的是，這是必要的。

我在創建視圖時遇到了一個有效的解決方案，但在我當前的設置中不支持此功能。

任何想法都受到高度贊賞。 謝謝！

Answer 1

嘗試這個：

select t1.* from attachments t1
left join attachments t2
on t1.entity_id = t2.entity_id and t1.group_id = t2.group_id and
   t1.version_number < t2.version_number
where t2.version_number is null

Answer 2

這可用於選擇所有

SELECT attachments.*
FROM (
    SELECT entitiy_id, group_id, MAX(version_number) AS max_version
    FROM attachments
    GROUP BY entitiy_id, group_id
) AS maxVersions
INNER JOIN attachments
ON attachments.entitiy_id = maxVersions.entitiy_id
AND attachments.group_id = maxVersions.group_id
AND attachments.version_number = maxVersions.max_version

將此擴展為僅查找單個entitiy_id只需要在子查詢中添加WHERE，這樣就可以了

SELECT attachments.*
FROM (
    SELECT entitiy_id, group_id, MAX(version_number) AS max_version
    FROM attachments
    WHERE entitity_id = [[YOUR ENTITIY ID HERE]]
    GROUP BY entitiy_id, group_id
) AS maxVersions
INNER JOIN attachments
ON attachments.entitiy_id = maxVersions.entitiy_id
AND attachments.group_id = maxVersions.group_id
AND attachments.version_number = maxVersions.max_version

如果你希望確保隨着行數的增加這種情況繼續快速運行，我建議你確保在行上添加一個密鑰(entitiy_id, group_id, max_version)因為子查詢可以依賴因此，確保它不會鎖定表。

Answer 3

這樣可以解決問題：

select a1.* from attachments a1
inner join ( select entitiy_id, group_id, max(version_number) as version_number
             from attachments
             group by entitiy_id, group_id) a2 on a1.entitiy_id = a2.entitiy_id and
                                                  a1.group_id = a2.group_id and
                                                  a1.version_number = a2.version_number

Answer 4

您還可以使用高性能通用表表達式（CTE）來解決此問題。

WITH CTE AS
(
SELECT entitiy_id, group_id, version_number, filename,       
ROW_NUMBER() OVER (PARTITION BY entitiy_id, group_id ORDER BY version_number DESC) as RowNum
FROM attachments
)
SELECT entitiy_id, group_id, version_number, filename
FROM CTE 
WHERE RowNum = 1

要么

SELECT T.entitiy_id, T.group_id, T.version_number, T.filename
FROM (SELECT entitiy_id, group_id, version_number, filename,       
     ROW_NUMBER() OVER (PARTITION BY entitiy_id, group_id ORDER BY version_number DESC) as RowNum
     FROM attachments
     ) as T 
WHERE RowNum = 1