MYSQL JOIN two tables limit results from second table by date

Question

I am trying to retrieve date from two tables using a MYSQL query. I want to join them together were categories.cat_id=topics.topic_cat. Multiple entries may have the same topic_cat, so I only want to SELECT the most recent, which is equal to MAX(topic_date).

The following query shows the correct information from topics, with only one result per topic_cat and that result having the most recent date.

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

Multiple rows may have the same value for topic_cat, but I only want to retrieve and join only the most recent, MAX(topic_date) and then join to a query which shows the following information from the categories table.

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;

This query displays the correct information, except one thing. It shows the topic_cat with the oldest entry, or MIN(topic_date). I have tried the following to get the topic_cat by newest entry or MAX(topic_date), but without success.

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

Any help or suggestions would be greatly appreciated.

Ok, so here is the sample data and associated desired result.

Table 1 = categories

_______________________________________________________
| cat_id | cat_name      | cat_description             |
-------------------------------------------------------
| 1      |  james        | Some information about james|
-------------------------------------------------------
| 2      |  myo          | Some information about myo  |
-------------------------------------------------------
| 3      |  brandon      | Some information about brandon  |
-------------------------------------------------------

Table 2 = topics

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

I would like the query to return the following data based on the above tables:

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

I hope that clarifies things.

Answer 1

Try This:

###
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);

Answer 2

Try changing

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

to:

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