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 wo
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: