Question

I have asked a similar question before and while the answers I got were spectacular I might need to clearify.

Just like This question I want to return N number of rows depending on a value in a column.

My example will be I have a blog where I want to show my posts along with a preview of the comments. The last three comments to be exact.

I have have I need for my posts but I am racking my brain to get the comments right. The comments table has a foreign key of post_id which obviously multiple comments can be attached to one post so if a post has 20 comments then I just want to return the last three. What makes this somewhat tricky is I want to do it in one query and not a ‘limit 3’ query per blog post which makes rendering a page with a lot of posts very query heavy.

SELECT * FROM replies GROUP BY post_id HAVING COUNT( post_id ) <=3 

This query does what I want but only returns one of each comment and not three.

Answer 1

SELECT  l.* FROM    (         SELECT  post_id,                 COALESCE(                 (                 SELECT  id                 FROM    replies li                 WHERE   li.post_id = dlo.post_id                 ORDER BY                         li.post_id, li.id                 LIMIT 2, 1                 ), CAST(0xFFFFFFFF AS DECIMAL)) AS mid         FROM    (                 SELECT  DISTINCT post_id                 FROM    replies dl                 ) dlo         ) lo, replies l WHERE   l.replies >= lo.replies         AND l.replies <= lo.replies         AND l.id <= lo.mid 

Having an index on replies (post_id, id) (in this order) will greatly improve this query.

Note the usage of l.replies >= lo.replies AND l.replies <= lo.replies: this is to make the index to be usable.

See the article in my blog for details:

• Advanced row sampling (how to select N rows from a table for each GROUP)