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Multi Threaded Comments PHP,[link widoczny dla zalogowanych]
I have a script I wrote I while back for comments,[link widoczny dla zalogowanych], but it is only single threaded. I would like it to be multi-threaded,[link widoczny dla zalogowanych], but only as so a user can reply to a comment,[link widoczny dla zalogowanych], not so a user can reply to a comment of a comment. So the threads would only be two deep.
Currently I store a comment_id against a user_id in my database,[link widoczny dla zalogowanych].
The only way I can think of to do the multi threaded comments,[link widoczny dla zalogowanych], is to have a parent field in the comments table. But if I do this then when I am selecting the comments with PHP,[link widoczny dla zalogowanych], I will have to do another SELECT command to select the comments children (if any) for each comment. Seems like a lot of work on the database.
There has to be a better way,[link widoczny dla zalogowanych]. Any ideas on this,[link widoczny dla zalogowanych]? Or tutorials?
A recursive query to select all comments based on their parent ids,[link widoczny dla zalogowanych]. This is supported by many database products and the syntax depends on the database type,[link widoczny dla zalogowanych]. Check the docs for more info (search for 'recursive'),[link widoczny dla zalogowanych].
If you store the article id in each (sub-)comment,[link widoczny dla zalogowanych], you can just select all comments with the article id in one regular select query. You can use the parent ids to properly display the comments on the page under the right parent comment:
SELECT * FROM comments WHERE article_id = :article_id
If you only need two levels of comments, you can use an extended where to include both first level and second level comments:
SELECT * FROM comments
WHERE parent_id = :article_id
OR parent_id IN (SELECT id FROM comments WHERE parent_id = :article_id)
It is also possible to use union all to combine two queries that have the same columns,[link widoczny dla zalogowanych], but since I assume that all data are from the same table,[link widoczny dla zalogowanych], there is probably no need for it (see the extended where-clause above):
SELECT * FROM comments WHERE parent_id = :article_id
UNION ALL
SELECT * FROM comments WHERE parent_id IN
(SELECT id FROM comments WHERE parent_id = :article_id)
Personally,[link widoczny dla zalogowanych], I would go for option 2 because it is simple (no exotic SQL construct required), efficient (1 query) and flexible (supports as many levels of comments as you like).
This is a common use for hierarchical,[link widoczny dla zalogowanych], or tree-structure data. I wrote a popular answer to this Stack Overflow question: What is the most efficient/elegant way to parse a flat table into a tree?
I also wrote a presentation describing alternatives for tree-structured data: Models for Hierarchical Data with SQL and PHP,[link widoczny dla zalogowanych].
Another solution that is not included in my presentation is the way Slashdot does threaded comments. They use a parent column like you do,[link widoczny dla zalogowanych], so each comment references the comment it replies to. But then they also include a root column so each comment knows the post it belongs to. There are seldom more than a few hundred comments on a given post,[link widoczny dla zalogowanych], and usually you want to get the whole tree of comments for a given post starting at the top of the comment thread:
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