MySQL query result fluctuating

Question

My problem is more clear on this post: Select all categories with latest post, user, and topic information

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I have a query that pulls a list of the categories for my forum along with the latest post in that category. The results come back as expected, except that sub_post information being pulled in the LEFT JOIN fp1 changes if if run the query several times.

I first noticed this problem when viewing my webpage and refreshing several times. The post that it is pulling fluctuates between 3 posts. I'm not sure how this is even possible, unless there is something wrong with the query.

Please, take a look at my query below and let me know if there is something I am doing wrong that might explain this odd behavior.

Cheers.

SELECT fc1.id AS cat_id, fc1.cat_name AS cat_name,
    fc1.cat_description AS cat_description, fc1.cat_views as cat_views, fp1.*
FROM forum_categories as fc1
LEFT JOIN (SELECT fp2.id AS sub_post_id,
                fp2.post_date as sub_post_date,
                fp2.post_topic as sub_post_topic,
                u2.id as sub_user_id, u2.username as sub_username,
                ft2.topic_subject as sub_topic_subject, ft2.topic_cat as sub_topic_cat
            FROM forum_posts as fp2
            LEFT JOIN users as u2 on fp2.post_by = u2.id
            LEFT JOIN forum_topics as ft2 on ft2.id = fp2.post_topic
            LEFT JOIN forum_categories as fcats on fcats.id = ft2.topic_cat
            ORDER BY fp2.id DESC)
as fp1 on fp1.sub_topic_cat = fc1.id
GROUP BY fc1.id; 

EXPLAIN SELECT:

+----+-------------+------------+--------+-------------------------+-------------+---------+--------------------+------+-------------+
| id | select_type | table      | type   | possible_keys           | key         | key_len | ref                | rows | Extra       |
+----+-------------+------------+--------+-------------------------+-------------+---------+--------------------+------+-------------+
|  1 | PRIMARY     | fc1        | index  | PRIMARY,cat_name_unique | PRIMARY     | 8       | NULL               |    3 | Using where |
|  1 | PRIMARY     | <derived2> | ref    | <auto_key0>             | <auto_key0> | 9       | tpw.fc1.id         |    9 | NULL        |
|  2 | DERIVED     | fp2        | index  | NULL                    | PRIMARY     | 8       | NULL               |   92 | NULL        |
|  2 | DERIVED     | u2         | eq_ref | PRIMARY                 | PRIMARY     | 8       | tpw.fp2.post_by    |    1 | NULL        |
|  2 | DERIVED     | ft2        | eq_ref | PRIMARY                 | PRIMARY     | 8       | tpw.fp2.post_topic |    1 | NULL        |
|  2 | DERIVED     | fcats      | eq_ref | PRIMARY                 | PRIMARY     | 8       | tpw.ft2.topic_cat  |    1 | Using index |
+----+-------------+------------+--------+-------------------------+-------------+---------+--------------------+------+-------------+

I have 3 tables: forums_categories, forums_topics, and forums_posts. I am trying to list the categories along with the latest post in that category. The forums_post is linked to forums_topics by a post_topic and the forums_topics is linked to the forums_categories with a topic_cat.

Answer 1

This was solved by _pala on this other question: https://stackoverflow.com/a/30048334/4864675

I was going about the query wrong which accounted for the odd behavior. Thanks _pala!

Here's the SQL that worked it out for me provided by user _pala:

select fc.cat_name, fc.cat_description, fc.cat_views, u.username, fp.post_date, ft.topic_subject
  from forum_categories fc
inner join forum_topics ft
  on fc.id = ft.topic_cat
inner join forum_posts fp
  on fp.post_topic = ft.id
inner join users u
  on fp.post_by = u.id
inner join (
  select topic_cat, max(fp.id) most_recent_post
    from forum_topics ft
      inner join forum_posts fp
        on fp.post_topic = ft.id
  group by topic_cat
) q
  on q.topic_cat = ft.topic_cat
    and fp.id = q.most_recent_post;

Answer 2

LEFT JOIN forum_categories as fcats on fcats.id = ft2.topic_cat

I believe the inclusion of that LEFT JOIN has no impact on the result other than to slow down processing. Remove it.

ORDER BY fp2.id DESC

That ORDER BY has no impact on the result because the GROUP BY will not care. Remove it.

If neither of those helps, then explain this:

The post that it is pulling fluctuates between 3 posts.

And please provide EXPLAIN SELECT ...