Question

I am making a nested comment system, and I have the following table:

create table comments (
id int unsigned primary key auto_increment,
author_id int unsigned not null,
target_id int unsigned not null,
target_type set('post','comment') not null,
content varchar(2000) not null
);

Each comment may be targeting a post, or another comment, in which case it would be considered a reply. Is there a simple way to count the number of nested comments (comments + replies + replies to replies, etc.) to a given post (say, a post with id 1)?

I know that if I had a field called target_post_id and another called target_comment_id, instead of having target_id and target_type, it would be very easy to count all the comments for any given post, but I have reasons for trying to keep this table structure, and truth be told, I’ve never dealt with recursive queries before. Any help is appreciated, thanks!!

Answer 1

The short answer is “no”. The slightly longer answer is that you could keep a comment count on the overall post, and update it as you add/delete comments in your nested structure (which is denormalising, but not entirely unexpected in this kind of situation); or as you point out you can always point back to the post from every comment.