我正在尝试使用 MYSQL 查询从两个表中检索日期。我想把他们一起加入到一起categories.cat_id=topics.topic_cat
。多个条目可能具有相同的topic_cat
,所以我只想选择最近的,等于MAX(topic_date)
。
以下查询显示来自主题的正确信息,每个 topic_cat 只有一个结果,并且该结果具有最近的日期。
SELECT topic_subject, topic_cat, topic_date
FROM topics
GROUP BY topic_cat DESC
多行可能具有相同的 topic_cat 值,但我只想检索和连接最近的 MAX(topic_date),然后加入一个查询,该查询显示类别表中的以下信息。
SELECT categories.cat_id, categories.cat_name, categories.cat_description, topics.topic_subject, topics.topic_cat, topics.topic_date, topics.topic_by
FROM categories
LEFT JOIN topics
ON categories.cat_id=topics.topic_cat
GROUP BY cat_id;
此查询显示正确的信息,除了一件事。它显示了 topic_cat 的最旧条目,或MIN(topic_date)
. 我已尝试以下方法通过最新条目或获取 topic_cat MAX(topic_date)
,但没有成功。
SELECT categories.cat_id, categories.cat_name, categories.cat_description
FROM categories
LEFT JOIN (SELECT topic_subject, topic_cat, topic_date, topic_by
FROM topics
GROUP BY topic_cat DESC) AS topics
ON categories.cat_id=topics.topic_cat
任何帮助或建议将不胜感激。
好的,这里是示例数据和相关的期望结果。
表 1 = 类别
_______________________________________________________
| cat_id | cat_name | cat_description |
-------------------------------------------------------
| 1 | james | Some information about james|
-------------------------------------------------------
| 2 | myo | Some information about myo |
-------------------------------------------------------
| 3 | brandon | Some information about brandon |
-------------------------------------------------------
表 2 = 主题
__________________________________________________
| topic_subject | topic_cat | topic_date | topic_by |
----------------------------------------------------------
| marcos | 2 | 2013-9-28 | User 1 |
---------------------------------------------------------
| ferdinand | 2 | 2013-9-29 | User 2 |
---------------------------------------------------------
| maria luisa | 2 | 2013-9-30 | User 1 |
---------------------------------------------------------
| Isabella | 1 | 2013-8-24 | User 3 |
--------------------------------------------------------
| Carlos | 3 | 2012-6-21 | User 2 |
--------------------------------------------------------
| Enrique | 3 | 2011-4-2 | User 3 |
---------------------------------------------------------
我希望查询根据上表返回以下数据:
_________________________________________________________________________________________________
| cat_id | cat_name | cat_description | topic_subject | topic_cat | topic_date | topic_by |
----------------------------------------------------------------------------------------------------------------
| 1 | james | Some information about james | Isabella | 1 | 2013-8-24 | User 3 |
----------------------------------------------------------------------------------------------------------------
| 2 | myo | Some information about myo | maria luisa | 2 | 2013-9-30 | User 1 |
----------------------------------------------------------------------------------------------------------------
| 3 | brandon | Some information about brandon | Carlos | 3 | 2012-6-21 | User 2 |
----------------------------------------------------------------------------------------------------------------
我希望这能澄清事情。