Left join is used to find rows that don't match. According to the image in your link, the data in topics.topic_id
and posts.post_topic columns are exactly same, thus matching one by one. That's why you could not get any
result. As Passionless pointed out, you can get the result by just joining two tables as follows:
$sql = "SELECT
from topics, posts,
where posts.post_topic=topics.topic_id //This might not be needed if the data in two columns are exactly same.
order by topics.topic_id DESC";