从具有唯一数据的多个表中排序MySQL SELECT数据

Question

In a group project I'm doing (a forum), I'm stuck on the task of sorting forum threads by the last reply. I know the title was somewhat confusing, but we'll try get an understanding of the scenario below.

I have two tables, which have some foreign keys and relation keys, and I want to order the rows selected from the table containing the threads by the newest post with the associated ID for that thread.

The two database tables looks like this:

- ForumThread -

• ID [int] , shows the unique identifier for a thread entity
• Title [string], the user-defined title of the thread
• Author [int], the ID of the user who made this thread (I know this is redundant though :P )
• Views [int], the amount of views on the thread
• Date [int], unix epoch number, defining the time the thread was created

- ForumPost -

• ID [int], the unique identifier for the thread reply (NOT the same as thread ID)
• ThreadID [int] , the ID referring to the thread this post/reply is associated with
• Poster [int], user ID of the user who made this reply
• Content [string], the content of the reply
• CreationDate [int] , a not-similarily-named key for when the reply was created

As you can see above, some of the table keys who should be able to relate to keys in the other table, have different names. Furthermore, I need to link/group/join ForumPost.ThreadID to ForumThread.ID, but all my current efforts have only led me to getting multiple rows for each reply to a thread, meaning that a thread with eg 2 replies would show up twice after sorting the result.

Here is what I have tried so far:

Attempts at sorting thread by last post:

    SELECT DISTINCT ForumPost.ThreadID, 
                    ForumThread.* 
      FROM ForumThread 
 LEFT JOIN ForumPost 
        ON ForumThread.ID = ForumPost.ThreadID 
  GROUP BY ForumPost.ThreadID 
  ORDER BY ForumPost.CreationDate DESC, 
           ForumThread.ID DESC 
     LIMIT $offset, 10

    SELECT ForumThread.* 
      FROM ForumThread 
 LEFT JOIN ForumPost 
        ON ForumThread.ID = ForumPost.ThreadID 
  GROUP BY ForumPost.ThreadID, 
           ForumThread.ID 
  ORDER BY ForumPost.CreationDate DESC, 
           ForumThread.ID DESC 
     LIMIT $offset, 10

    SELECT DISTINCT ForumThread.* 
      FROM ForumThread 
INNER JOIN ForumPost 
        ON ForumThread.ID = ForumPost.ThreadID 
  GROUP BY ForumThread.ID 
  ORDER BY ForumPost.CreationDate DESC, 
           ForumThread.ID DESC 
     LIMIT $offset, 10

Original simple query:

    SELECT * 
      FROM ForumThread 
  ORDER BY ID DESC 
     LIMIT $offset, 10

Would you in this case just recommend grabbing the information separately, and use a language such as PHP (obviously this is what we're using for this project) to associate the two different MySQL results and sort them then, or do you have a faster and more compact solution?

Note; For all those of you who recommend me to use MySQLi, I can confirm that my group have pondered whether to use MySQLi or MySQL, though our learning pensum have only covered the outdated MySQL material, making it weight out the possibility for learning MySQLi with our narrow deadline for this project.

Answer 1

This is a possibility using a correlated subquery; hope it helps

SELECT 
   ForumThread.id, Title, 
      (SELECT MAX(CreationDate) FROM ForumPost 
       WHERE 
           ForumPost.ThreadID = ForumThread.id) AS lastentry
 FROM ForumThread group by 1, 2 order by 3 desc;