Question

I am building a forum for an application and I am stuck trying to get the latest poster on a discussion. The database I am using is Oracle.

I have the following tables:

discussions
    id 
    course_id
    user_id
    title
    stub
    created_at

threads
   id
   discussion_id
   user_id
   created_at
   updated_at
   message

 discussion_views
     discussion_id
     user_id
     time

 users
     id
     username

And I have the following query:

select discussions.created_at, 
       discussions.title, 
       users.username, 
       count(threads.id) AS "replies", 
       count(distinct discussion_views.discussion_id) AS "views" 
 from discussions
 left join threads on discussions.id=threads.discussion_id
 left join discussion_views on discussions.id=discussion_views.discussion_id
 join users on users.id=discussions.user_id
 group by discussions.created_at, discussions.title, users.username
 order by discussions.created_at desc

What I need is to get the last user who posted in a thread and the date. Should I make an inner insert, a join or should I make another insert. I also want my query to be performant because my forum will need to handle a lot of traffic.

Answer 1

Try:

select * from
(select discussions.created_at, 
        discussions.title, 
        users.username as discussion_created_by, 
        count(distinct threads.id) over (partition by discussions.created_at, 
                                                      discussions.title, 
                                                      users.username) AS replies, 
        count(distinct discussion_views.time) 
             over (partition by discussions.created_at, 
                                discussions.title, 
                                users.username) AS "views",
        threads.user_id AS latest_post_by,
        threads.updated_at AS latest_post_at,
        row_number() over (partition by discussions.created_at, 
                                        discussions.title, 
                                        users.username
                           order by threads.id desc) AS rn
 from discussions
 left join threads on discussions.id=threads.discussion_id
 left join discussion_views on discussions.id=discussion_views.discussion_id
 join users on users.id=discussions.user_id) sq
where rn=1
order by created_at desc