MYSQL JOIN兩個表限制結果從第二個表到日期

我想從兩個表中使用MYSQL查詢檢索日期。我想一起加入他們分別是categories.cat_id=topics.topic_cat。多個條目可能有相同的topic_cat，所以我只想選擇最近的，這相當於MAX(topic_date)。MYSQL JOIN兩個表限制結果從第二個表到日期

以下查詢顯示來自主題的正確信息，每個topic_cat只有一個結果，而結果具有最近的日期。

SELECT topic_subject, topic_cat, topic_date 
FROM topics 
GROUP BY topic_cat DESC

多行可能有topic_cat相同的值，但我只想要檢索並加入只有最近，MAX（topic_date），然後加入一個查詢，顯示從類別表以下信息。

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;

此查詢顯示正確的信息，除了一件事情。它顯示了具有最舊條目的topic_cat，或MIN(topic_date)。我已經嘗試通過以下方式獲得topic_cat的最新條目或MAX(topic_date)，但沒有成功。

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

任何幫助或建議將不勝感激。

好了，所以這裏是樣品的數據和相關聯的期望的結果。

表1 =類別

_______________________________________________________ 
| 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 =主題

__________________________________________________ 
| 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  | 
---------------------------------------------------------

我想查詢返回基於上述表中的以下數據：

_________________________________________________________________________________________________ 
| 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   | 
----------------------------------------------------------------------------------------------------------------

我希望澄清事情。

來源

2013-09-30 Jimbob

請提供採樣數據和相關聯的期望的結果。 –

嘗試使用'LIMIT'子句。 –

我可以限制最近的日期嗎？ – Jimbob

嘗試：

### 
SELECT * FROM categories c 
LEFT JOIN topics t ON c.cat_id = t.topic_cat 
WHERE c.cat_id IN (SELECT t1.cat_id FROM (
    SELECT c.cat_id, c.cat_name, MAX(t.topic_date) AS maxdate FROM categories c 
    LEFT JOIN topics t ON c.cat_id = t.topic_cat 
    GROUP BY c.cat_name 
) as t1 WHERE t1.maxdate = t.topic_date OR t.topic_date IS NULL); 

### without nulls 
SELECT * FROM categories c 
    LEFT JOIN topics t ON c.cat_id = t.topic_cat 
    WHERE c.cat_id IN (SELECT t1.cat_id FROM (
    SELECT c.cat_id, c.cat_name, MAX(t.topic_date) AS maxdate FROM categories c 
    LEFT JOIN topics t ON c.cat_id = t.topic_cat 
    GROUP BY c.cat_name 
) as t1 WHERE t1.maxdate = t.topic_date);

來源

2013-09-30 19:57:08 Jason

嘗試改變

LEFT JOIN (SELECT topic_subject, topic_cat, topic_date, topic_by 
FROM topics 
GROUP BY topic_cat DESC) AS topics

到：

LEFT JOIN (SELECT topic_subject, topic_cat, topic_date, topic_by 
FROM topics 
GROUP BY topic_cat 
ORDER BY topic_date DESC 
LIMIT 0,1) AS topics

來源

2013-09-30 05:23:32

MYSQL JOIN兩個表限制結果從第二個表到日期

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