Question

I’m not even sure this is possible to do efficiently, but here’s my problem:

I’m writing what’s essentially a blog engine where a blog post and all replies to each blog post can tagged.

So, I could have a blog post tagged ‘stack’, and a reply to that post tagged ‘overflow’.

Right now, I’m trying to generate a list of the most popular tags when a user hits a special page in my application. It should return not only the n most popular tags by descending number of blog posts, but also the number of blog posts associated with each tag, even if a reply in that post but not the post itself is tagged with that tag.

So, if BlogPost A is tagged with ‘foo’, and a reply in BlogPost B is tagged with ‘foo’, the popular tag summary should count that as two blog posts in total, even though BlogPost B is not technically tagged.

Here’s a description of the tables/fields that might be relevant:

BlogPosts | id     # Primary key for all tables, Rails-style  BlogComments | id | blog_post_id  Tags | id | name   # 'foo'  Taggings | id | tag_id | blog_post_id | blog_comment_id 

There’s some denormalization in Taggings for the sake of convenience. If someone tags BlogPost, it fills in the blog_post_id field, and blog_comment_id remains NULL. If someone tags a comment to a post, it fills in both blog_post_id and blog_comment_id.

Is there some way to return a sorted list of the most popular tags in one or several SQL queries? I’m thinking I might need to just run a computationally-expensive script every few minutes on a cron job and render the cached output instead of running this every time somebody hits the page…

Thanks!

Answer 1

So far I see nothing complicated in your request:

SELECT   tag_id,   COUNT(blog_post_id) + COUNT(blog_comment_id) tag_count FROM   Taggings GROUP BY   tag_id ORDER BY   COUNT(blog_post_id) + COUNT(blog_comment_id) DESC 

If you want to count ‘affected blog posts’ only, I think that’s the way:

SELECT   t.id    tag_id,   t.name  tag_name,   COUNT(DISTINCT COALESCE(x.blog_post_id, c.blog_post_id)) tag_count FROM   Tags                    t     INNER JOIN Taggings     x ON x.tag_id = t.id   LEFT  JOIN BlogComments c ON c.id     = x.blog_comment_id GROUP BY   t.id,   t.name ORDER BY   COUNT(DISTINCT COALESCE(x.blog_post_id, c.blog_post_id)) DESC