How to Efficiently Retrieve the Latest Post for Each Author in MySQL?

Retrieving Latest Post per Author in MySQL: Using JOIN Instead of Order by Before Group By

When striving to retrieve the latest post for each author in a database, the initial approach using an ORDER BY clause before GROUP BY in MySQL often yields undesirable results. However, employing a subquery to order rows before grouping can be a potential solution.

Why Order by Before Group By Can Be Problematic

Consider the example 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

While this query aims to group posts by author and return the latest post for each, it can fail when there are multiple posts with the same post_date. In such cases, the order in which posts are returned within each author group becomes unpredictable, leading to inconsistent results.

Using a Subquery to Solve the Issue

The proposed solution to this issue is to utilize a subquery as follows:

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

By using this subquery, posts are first ordered by their post_date in descending order. The resulting subquery table then serves as the source for the main query, ensuring that the latest posts are placed at the top of each author group before grouping.

An Alternative Approach with JOIN

Another effective approach to retrieve the latest post per author is to utilize a JOIN operation. The following query demonstrates this method:

SELECT p1.* 
FROM wp_posts p1
INNER JOIN
(
    SELECT max(post_date) MaxPostDate, post_author
    FROM wp_posts
    WHERE post_status='publish'
       AND post_type='post'
    GROUP BY post_author
) p2
  ON p1.post_author = p2.post_author
  AND p1.post_date = p2.MaxPostDate
WHERE p1.post_status='publish'
  AND p1.post_type='post'
order by p1.post_date desc

This query involves two steps:

1. A subquery finds the maximum post_date and corresponding post_author for each author.
2. The main query joins the table containing posts with the subquery result on both post_date and post_author, effectively retrieving the latest post for each author.

Conclusion

While using a subquery to order rows before grouping can be an effective solution to retrieve the latest post per author, it is not the only approach available. The JOIN operation provides an alternative method that is both efficient and produces reliable results.

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