Retrieve 2 last posts for each category

问题

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.

回答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

回答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: