Retrieve 2 last posts for each category

Lets say I have 2 tables: blog_posts and categories. Each blog post belongs to only ONE category, so there is basically a foreign key between the 2 tables here.

I would like to retrieve the 2 lasts posts from each category, is it possible to achieve this in a single request? GROUP BY would group everything and leave me with only one row in each category. But I want 2 of them.

It would be easy to perform 1 + N query (N = number of category). First retrieve the categories. And then retrieve 2 posts from each category.

I believe it would also be quite easy to perform M queries (M = number of posts I want from each category). First query selects the first post for each category (with a group by). Second query retrieves the second post for each category. etc.

I'm just wondering if someone has a better solution for this. I don't really mind doing 1+N queries for that, but for curiosity and general SQL knowledge, it would be appreciated!

Thanks in advance to whom can help me with this.

Check out this MySQL article on how to work with the top N things in arbitrarily complex groupings; it's good stuff. You can try this:

SET @counter = 0;
SET @category = '';

SELECT
  *
FROM
(
  SELECT
    @counter := IF(posts.category = @category, @counter + 1, 0) AS counter,
    @category := posts.category,
    posts.*
    FROM
      (
      SELECT
        *
        FROM test
        ORDER BY category, date DESC
      ) posts
) posts
HAVING counter < 2

SELECT  p.*
FROM    (
        SELECT  id,
                COALESCE(
                (
                SELECT  datetime
                FROM    posts pi
                WHERE   pi.category = c.id
                ORDER BY
                        pi.category DESC, pi.datetime DESC, pi.id DESC
                LIMIT 1, 1
                ), '1900-01-01') AS post_datetime,
                COALESCE(
                (
                SELECT  id
                FROM    posts pi
                WHERE   pi.category = c.id
                ORDER BY
                        pi.category DESC, pi.datetime DESC, pi.id DESC
                LIMIT 1, 1
                ), 0) AS post_id
        FROM    category c
        ) q
JOIN    posts p
ON      p.category <= q.id
        AND p.category >= q.id
        AND p.datetime >= q.post_datetime
        AND (p.datetime, p.id) >= (q.post_datetime, q.post_id)

Make an index on posts (category, datetime, id) for this to be fast.

Note the p.category <= c.id AND p.category >= c.id hack: this makes MySQL to use Range checked for each record which is more index efficient.

See this article in my blog for a similar problem: