How to Order MySQL Results Before Grouping?

MySQL: Ordering Results Before Group By

When grouping results in MySQL, it's important to understand the order of operations. By default, MySQL will order the results after they have been grouped, which may not be the desired behavior in certain cases.

Original Query and Limitation

Consider the following query:

SELECT wp_posts.* FROM wp_posts
WHERE wp_posts.post_status='publish'
AND wp_posts.post_type='post'
GROUP BY wp_posts.post_author
ORDER BY wp_posts.post_date DESC

This query correctly groups the results to show only the latest post for each author. However, the results are ordered after the grouping, so the latest post may not necessarily be at the beginning of the list.

Solution Using HAVING

To order the results before grouping, we can use the HAVING clause:

SELECT wp_posts.* FROM wp_posts
WHERE wp_posts.post_status='publish'
AND wp_posts.post_type='post'
GROUP BY wp_posts.post_author
HAVING wp_posts.post_date = MAX(wp_posts.post_date)
ORDER BY wp_posts.post_date DESC

The HAVING clause filters the results after the grouping, ensuring that only the rows with the maximum post_date for each author are returned. This guarantees that the latest post is shown at the beginning of the list.

Alternative Solution for Other Databases

If using a database that does not support the HAVING clause (such as Postgres or SQL Server), an alternative solution is to join the original table with a subquery that computes the maximum post_date for each author:

SELECT
    wp_posts.*
FROM
    wp_posts
    JOIN
    (
        SELECT
            g.post_author
            MAX(g.post_date) AS post_date
        FROM wp_posts as g
        WHERE
            g.post_status='publish'
            AND g.post_type='post'
        GROUP BY g.post_author
    ) as t
    ON wp_posts.post_author = t.post_author AND wp_posts.post_date = t.post_date
ORDER BY wp_posts.post_date

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