SQL joining Thread with post issue how would you solve it - sql

Question

I need help
$sql = "SELECT
topics.topic_id,
topics.topic_subject,
topics.topic_date,
topics.topic_cat,
topics.topic_by,
topics.topic_open_close,
topics.topic_pin
FROM
topics
LEFT JOIN posts ON topics.topic_id = posts.post_topic_id
WHERE
topics.topic_cat = '". $_GET['id']."' AND topics.topic_pin = '0' ORDER BY posts.post_date DESC;
my SQL is ORDERING the Topics base on post date. the problem I'm having is that when I reply more than 1 time, lets say 3 replies, it shows 3 of the same threads.
the purpose I'm trying to archive with this Query is to bump up the thread by posting base on post_date. its doing the job. It's its just showing more than one thread.
how would you guys get write this Query to do the job?

Answer 1

Seems you are joining the posts table even thought you are not displaying some of its columns. But if you want to display 1 topic based on the latest post date, you can join the max(post_date) via subquery.
SELECT
topics.topic_id,
topics.topic_subject,
topics.topic_date,
topics.topic_cat,
topics.topic_by,
topics.topic_open_close,
topics.topic_pin
FROM topics
INNER JOIN
(SELECT max(post_date) post_date, post_topic_id FROM posts GROUP BY post_topic_id) t2 ON topics.topic_id = t2.post_topic_id
WHERE
topics.topic_cat = '". $_GET['id']."' AND topics.topic_pin = '0'

Answer 2

I solved it the long way. I needed to get it done. My plan was to make it short.
with a SQL Query.
I have a functions.php folder to run all my functions thru out the project.
so i created a function inside the functions.php to run all query and not having to repeat myself.
examp:
function query($sql, $conn){
$result = mysqli_query($conn, $sql);
if(!$result){
echo 'There is a Error in the SQL '. mysqli_error($conn);
}else{
return $result;
}
}
I added a column on Topics called "Topic_last_post_date". Every time someone post if the Query to post the post goes successfully it will execute the query to update the "Topic_last_post_date" on the assigned topic.
//posting Query ect ect..
if(!$result_of_query){
echo'Error';
}else{
$stamp_post_date_on_Topic = query("UPDATE topics SET topic_last_post_date = NOW() WHERE topic_id='".$topicid."' ", $conn);}
then I ended using the this query to display the topic DESC = Descending starting from the last to "Topic_last_post_date" posted.
$sql = "SELECT
topic_id,
topic_subject,
topic_date,
topic_cat,
topic_by,
topic_open_close,
topic_pin,
topic_last_post_date
FROM
topics
WHERE
topic_cat = '". $_GET['id']."' AND topic_pin = '0' ORDER BY topic_last_post_date DESC LIMIT ".$offset.", ".$no_of_records_per_page." ";
I hope it help someone! thanks guys