How to select 3 topic and list the most recent comment

Databases
1

I been banging my head against the wall. I’m not entirely sure what i am doing wrong here…

I hope some SQL guru is out there and willing to help.

I have two tables. Discussion_Topic and Discussion_Comment.
There is a dtop_id value in the ‘discussion_topic’ table that matches a column in ‘discussion_comment’ called ‘dcom_topic_id’. (that allows for selecting comments that belong to the topic with matching IDs. Pretty simple.)

I made a new column in discussion_topic called ‘rank’. There will be 3max listed and they are labeled 1,2,3 and in that order too. So i can grab 3 topic titles and output them to the page. (not having a problem with this)

Next I want to grab the most recent comment from the comments table and output the comment just below each topic title. And Here lies the issue. The results always show the oldest comment.

Here is the SQL statement i am using:


SELECT * FROM discussion_topic, discussion_comment
WHERE discussion_topic.dtop_id = discussion_comment.dcom_topic_id
AND discussion_topic.rank > 0
AND discussion_topic.dtop_active = 1
GROUP BY discussion_topic.dtop_id
ORDER BY discussion_topic.jtcrank ASC

I think this is probably just something simple that i am doing wrong.

thanks in advance!

2

Don’t know if it works, but you could try


ORDER BY 
    discussion_topic.jtcrank ASC
  , discussion_comment.dcom_id DESC

assuming that the comments table has a autoincrement id column as well :slight_smile:

3

I think i tried that before…I’ll try it again, but i don’t think it worked for some reason.

4

Yep, tested that and it doesn’t sort the second parameter there. dcom_id and jtcrank are both INT – however at first, the jtcrank was TINYINT so i did an alter table and made it an INT.

Should i delete that column and remake it?

5

i think the two fields that you ORDER BY have to match there type? Is that right? Welp, I deleted the jtcrank column and added it back. I made 3 topics 1,2,3. Does it matter if PRECISION is set to 11 for jtcrank field vs. PRECISION is set to “2” on the field I am comparing it too?

6 

SELECT * 
FROM discussion_topic AS dt
INNER JOIN discussion_comment AS dc1
ON dt.dtop_id = dc1.dcom_topic_id
INNER JOIN
  (SELECT 
       dcom_topic_id
     , MAX(dcom_id) AS max_dcom_id
   FROM discussion_comment
   GROUP BY dcom_topic_id
  ) AS dc2
ON  dc2.dcom_topic_id = d1.dcom_topic_id
AND dc2.max_dcom_id = d1.dcom_id
WHERE dt.rank > 0
AND   dt.dtop_active = 1
GROUP BY dt.dtop_id
ORDER BY dt.jtcrank ASC

Maybe you can even eliminate the final GROUP BY dt.dtop_id, because I think the query will give only one row for each dtop_id anyway.

7

What do you mean? The fields you order by don’t have to be all of the same type.

8

I read that in order to do a ORDER a,b.

a and b have to match.

But that’s all it said. It didnt say in what way it needed to match. So, who knows. I was not sure as that statement was kinda confusing.

9

i will try out the SQL statement – looks like a winner to me!

10

hey man, thanks alot! That worked great!

11

flat out wrong

whatever you were reading, throw it away

:slight_smile:

12

haha, well it was on the internet – if i find the article again, I’ll post it here.

it wasn’t really very clearly what they meant by “match”.