Given two tables, select all data from one table and only the most recent from the other

Question

I'm trying to build a forum website that uses PHP and a MySQL database to store its category and topic content. On the homepage, I want to have a table that shows a list of all the categories and the most recent posted topic from each category.

I want to write a query that returns all of the categories from the category table and only the most recent topics for each category from the topics table.

My tables look like this:

categories

+--------+-------------+--------------------------+
| cat_id |  cat_name   |     cat_description      |
+--------+-------------+--------------------------+
|      1 | Automobiles | *Tim Taylor manly grunt* |
|      2 | English     | How to English           |
|      3 | Gardening   | Lawn and Order           |
+--------+-------------+--------------------------+

topics

+----------+-----------------------+------------------------+-----------+----------+
| topic_id |      topic_name       |     topic_content      | topic_cat | topic_by |
+----------+-----------------------+------------------------+-----------+----------+
|        1 | '67 Chevy Question    | Weird knocking noise?  |         1 |        1 |
|        2 | You're vs Your        | What's the difference? |         2 |        3 |
|        3 | Jumper cables?        | The proper hookup      |         1 |        2 |
|        4 | '03 Pathfinder        | Next newest model?     |         1 |        1 |
|        5 | There, Their, They're | Know the difference    |         2 |        4 |
+----------+-----------------------+------------------------+-----------+----------+

I found a relevant answer on https://stackoverflow.com/a/12586263/7249891 under Trick #3, but after a couple of hours of fiddling, am unable to boil it down to a query that works for me.

My question is, how do I adjust my original query

SELECT c.cat_name AS Category, t.topic_cat AS Recent Topic 
FROM categories c
JOIN topics t
WHERE c.cat_id = t.topic_cat

so it returns all the categories in the database, but only the most recent topic from each category in a result similar to this

+-------------+-----------------------+
|  Category   |     Recent Topic      |
+-------------+-----------------------+
| Automobiles | '03 Pathfinder        |
| English     | There, Their, They're |
| Gardening   | NULL                  |
+-------------+-----------------------+

Clarifications: In this forum, there are several categories created by admins that any user can post a topic in.

In a topic, the topic subject is a question asked by a user and the topic content is additional information about that question.

Cat_id and topic_id are both auto incrementing primary keys.

Topic_subject is a foreign key that references cat_id.

Assume that the most recent topic in the topics table is the one with the highest topic_id number because of the primary key behavior. There is also a date field in this table (which I realized last minute I forgot to include here).

There are two other tables I didn't list here: a users and replies table. Topic_by is a foreign key that references the users table.

If there are no topics in a category (the gardening category in my above example), we'll assume the PHP portion of the program will make that part of the list say "(none)".

CLAbeel · Accepted Answer · 2016-12-05T19:01:26.427

0

First find the latest post in each category:

select topic_cat, max(topic_id) as latest_topic
from topics group by topic_cat

Then add that to your join conditions:

SELECT  c.cat_name AS Category, t.topic_name AS Recent_Topic 
FROM categories c
left JOIN topics t on c.cat_id = t.topic_cat 
left join (select topic_cat, max(topic_id) as latest_topic
        from topics group by topic_cat) as latest_topics 
        on latest_topics.topic_cat = c.cat_id
        and latest_topics.latest_topic = t.topic_id 
where latest_topics.topic_cat is not null or t.topic_cat is null;

edited Dec 05 '16 at 19:01

answered Dec 05 '16 at 18:15

CLAbeel

1,078
14
20

Thanks, this gave me the results I was after. I've been trying to break the query down to fully understand what's going on, but I'm still pretty new to the concept of a SELECT within a SELECT. – MrZander Dec 06 '16 at 20:44
Did you try just running `select topic_cat, max(topic_id) as latest_topic from topics group by topic_cat`? Think of those results as another table, that you are then joining to your other "normal" tables. – CLAbeel Dec 06 '16 at 23:10

Ullas · Answer 2 · 2016-12-05T20:00:02.263

First select the row from topics tables having the max id group by topic_cat and do a left join with categories table.

Query

select a.`cat_name`, 
       b.`topic_name` as `Recent Topic`
from `categories` a
left join(
  select t1.`topic_id`, 
         t1.`topic_name`, 
         t1.`topic_content`, 
         t1.`topic_cat`, 
         t1.`topic_by` from 
  (
    select `topic_id`, 
           `topic_name`, 
           `topic_content`, 
           `topic_cat`, 
           `topic_by`,
    (
        case `topic_cat` when @curA 
        then @curRow := @curRow + 1 
        else @curRow := 1 and @curA := `topic_cat` end 
    ) as `rn` 
    from `topics` t, 
    (select @curRow := 0, @curA := '') r 
    order by `topic_cat`, `topic_id` desc 
    )t1 
    where t1.`rn` = 1
)b
on a.`cat_id` = b.`topic_cat`;

SQL Fiddle Demo

Given two tables, select all data from one table and only the most recent from the other

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