mysql - 如何返回 GROUP BY 语句的最新记录

Question

我有一个表 (call_history)，其中针对另一个表 (call_detail) 中的单个 ID 有多个记录。我试图返回一个结果，该结果将为每个 ID 提供一行，但最新条目除外。

例如，根据单个 ID 将所有行分组，但仅返回 Updated_at 字段(这是一个日期字段)的最新行。

到目前为止我的查询...

SELECT MAX(cd.id) as id, cd.first_name, cd.summary, cd.due_at, ch.body, ch.updated_at 
FROM call_detail as cd 
LEFT JOIN call_history as ch on cd.id = ch.ticket_id 
WHERE cd.status = 'open' AND (NOW() > due_at) 
GROUP BY cd.id HAVING COUNT(*) > 1 
ORDER BY cd.due_at DESC

...返回我想要的“种类”，但它为我提供了 update_at 字段中最旧的条目。我需要相反的方式。

更新

我的表结构如下:

调用详细信息

id | summary | description | due_at              | first_name | last_name
1  | Call 1  | some text   | 20/02/2014 17:00:00 | Joe        | Bloggs
2  | Call 2  | some text   | 18/02/2014 15:00:00 | Fred       | Durst
3  | Call 3  | some text   | 02/03/2014 01:00:00 | Joe        | Bloggs

通话历史记录

id | ticket_id | body      | updated_at          | first_name | last_name
1  | 1         | update 1  | 17/02/2014 16:00:00 | Joe        | Bloggs
2  | 1         | update 2  | 17/02/2014 16:02:00 | Fred       | Durst
3  | 2         | update 1  | 16/02/2014 12:02:00 | Tom        | Thumb
4  | 1         | update 3  | 17/02/2014 16:10:00 | Joe        | Bloggs
5  | 2         | update 2  | 17/02/2014 01:02:00 | Jack       | Reacher

等等...

我需要检索以下输出:

ticket_id | summary | due_at              | first_name | body     | updated_at
1         | Call 1  | 20/02/2014 17:00:00 | Joe        | Update 3 | 17/02/2014 16:10:00
2         | Call 2  | 18/02/2014 15:00:00 | Fred       | Update 2 | 17/02/2014 01:02:00

Answer 1

尝试这个查询。您需要 call_history 中的最新记录，因此首先应使用这些日期创建子查询(请参阅 CH_MAX 子查询)，然后使用 ticket_id 进行 JOIN 并updated_at:

SELECT cd.id as id, 
       cd.first_name, 
       cd.summary, 
       cd.due_at, 
       ch.body, 
       ch.updated_at 
FROM call_detail as cd 

LEFT JOIN 
 ( SELECT ticket_id, MAX(updated_at) as max_updated_at 
          FROM call_history 
          GROUP BY ticket_id
  ) as CH_MAX ON cd.id = CH_MAX.ticket_id

LEFT JOIN call_history as ch ON cd.id = ch.ticket_id 
                                AND CH_MAX.max_updated_at = ch.updated_at

WHERE cd.status = 'open' AND (due_at<NOW()) 
ORDER BY cd.due_at DESC

SQLFiddle demo