Question

Okay so I have two tables, a category table and a posts table.

I was hoping to construct a query that would return each category, the last record in the post table for that category (category.id = post.category_id) AND the rowcount of all posts with post.category_id = category.id)

i.e

A list of categories, each with the record of the latest post for the category and the number of posts in that category.

I’ve tried various joins and subselects, and I have managed to get a list of categories, their post count, and their FIRST post (I need the last).

I have also managed to get a list of categories and their LAST post, but not the post count.

Any help/directions will be much appreciated!

Answer 1

Try this:

SELECT category_id, MAX(post_id) AS post_id, COUNT(*) AS posts
FROM post
GROUP BY category_id

This assumes that you have a column post_id that is strictly increasing and unique.

If you have no such field but you have for example post_date that is not unique, you may get two rows for some categories in rare cases where two posts have exactly the same timestamp.

To also get categories without posts use a LEFT JOIN:

SELECT
    category.id AS category_id,
    MAX(post.id) AS post_id,
    COUNT(post.category_id) AS posts
FROM category
LEFT JOIN post
ON category.id = post.category_id
GROUP BY category.id

If you need more columns from each table you can join the result above with the original tables:

SELECT *
FROM
(
    SELECT
        category.id AS category_id,
        MAX(post.id) AS post_id,
        COUNT(post.category_id) AS posts
    FROM category
    LEFT JOIN post
    ON category.id = post.category_id
    GROUP BY category.id
) T1
JOIN category
ON T1.category_id = category.id
LEFT JOIN post
ON T1.post_id = post.id