How to build Threaded comments with a 1 or 2 queries?

Question

Can anyone suggest a creative database structure + fetching algorithm for a threaded comments system, that would output x amount of threads per page (with unlimited replies for each)?

I can run a query to get the threads, and in each instance of a loop, run another query to echo out the replies.... but that's a bad idea.

Answer 1

see this - http://stackoverflow.com/questions/1260878/mysql-structure-for-comments-and-comment-replies

Answer 2

Add two columns to the comment table: parentCommentId and rootCommentId.

parentCommentId is the id of the parent comment, and rootCommentId is the id of the comment that started this thread.

To display N threads, you'll need two queries:

1. Get N rows from the comment table where rootCommentId = id
2. Get all comments for these N threads

(You can combine these two into a single GroupBy query.)

Answer 3

If you need only 2 levels, here's a way with one query:

Your table - id, parent_id, comment columns

Code

$rows = mysql_query('
  select *
  FROM
    comments
  ORDER BY
    id DESC');

$threads = array();
foreach($rows as $row) {
  if($row['parent_id'] === '0') {
    $threads[$row['id']] = array(
      'comment' => $row['comment'],
      'replies' => array()
    );
  } else {
    $threads[$row['parent_id']]['replies'][] = $row['comment'];
  }
}

In $threads you will have all your main threads and $threads[$id]['replies'] holds all replies. The threads are sorted - latest = first, add some paging and you're good to go.