Question

I am trying to code a forum website and I want to display a list of threads. Each thread should be accompanied by info about the first post (the “head” of the thread) as well as the last. My current database structure is the following:

threads table:
id   - int, PK, not NULL, auto-increment
name - varchar(255)

posts table:
id        - int, PK, not NULL, auto-increment
thread_id - FK for threads

The tables have other fields as well, but they are not relevant for the query. I am interested in querying threads and somehow JOINing with posts so that I obtain both the first and last post for each thread in a single query (with no subqueries). So far I am able to do it using multiple queries, and I have defined the first post as being:

SELECT *
FROM threads t
LEFT JOIN posts p ON t.id = p.thread_id
ORDER BY p.id
LIMIT 0, 1

The last post is pretty much the same except for ORDER BY id DESC. Now, I could select multiple threads with their first or last posts, by doing:

SELECT *
FROM threads t
LEFT JOIN posts p ON t.id = p.thread_id
ORDER BY p.id
GROUP BY t.id

But of course I can’t get both at once, since I would need to sort both ASC and DESC at the same time.
What is the solution here? Is it even possible to use a single query? Is there any way I could change the structure of my tables to facilitate this? If this is not doable, then what tips could you give me to improve the query performance in this particular situation?

Answer 1

You could do something with a subquery and joins:

SELECT first.text as first_post_text, last.text as last_post_text
FROM
  (SELECT MAX(id) as max_id, MIN(id) as min_id FROM posts WHERE thread_id = 1234) as sub
JOIN posts first ON (sub.max_id = first.id)
JOIN posts last ON (sub.min_id = last.id)

But that doesn’t solve your problem of doing it without subqueries.

You could add columns to your threads table so that you keep the id of the first and last post of each thread. The first post would never change, but every time you added a new post you would have to update that record in the threads table, so that would double your writes, and you may need to use a transaction to avoid race conditions.

Or you could go so far as to duplicate information about the first and last post in the threads row. Say you needed the user_id of the poster, the timestamp it was posted, and the first 100 characters of the post. You could create 6 new columns in the threads table to contain those pieces of data for the first and last post. It duplicates data, but it means you may be able to display a list of threads without needing to query the posts table at all.