SQL：与另一个表中的最新行联接

Question

I'm trying to build a simple forum as a learning exercise. At the moment I have a topics(questions) table and a posts table (where the post_thread_id refers to the topic id).

I want to display the topics in the order by which the most recent post in that topic appears. Looking online I used this command:

SELECT topics.topic_title, topics.topic_date, topics.topic_title, max( posts.post_date )
FROM topics
LEFT OUTER JOIN posts ON topics.topic_id = posts.post_thread_id
GROUP BY topics.topic_id
ORDER BY MAX( posts.post_id ) ASC

which seems to work. But it doesn't work if there are no posts linked to a topic (ie an empty topic) as NULL comes first every time. I want it so that if no posts exist then the topic date is used to order.

How do I select the topic date if there are no posts?

Thanks (I'd appreciate a simple explanation as to how it is working as I am trying to learn).

Answer 1

ORDER BY 
CASE WHEN MAX( posts.post_id ) IS NULL THEN 1 ELSE 0 END ASC

Self explanatory,when MAX(post_Id) is null it will change the order priority.