Retrieve 2 last posts for each category - sql

Question

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.

Answer 1

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

Answer 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:
MySQL: emulating ROW_NUMBER with multiple ORDER BY conditions