MySQL分組前如何依日期排序？

MySQL Order before Group By

The Problem:

To find the latest post for each author and group the results, we need to order the posts by date before applying the group by operation. However, the given query is ordering the results after grouping, which can lead to incorrect results.

The Solution:

To fix this issue, we can use the HAVING clause instead of ORDER BY within the group by operation. The HAVING clause allows us to apply conditions to the aggregated data and exclude groups that do not meet the condition.

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;

By placing the condition in the HAVING clause, we ensure that the results are filtered before the group by operation is applied. This guarantees that each author has only their latest post included in the results.

Additional Considerations:

This method of ordering before grouping is specific to MySQL and may not work in other databases such as Postgres or SQL Server. For these databases, alternative approaches, such as using a subquery or joining with a maximum value table, may be necessary.

However, it's important to note that using the GROUP BY with ordering can be efficient for large datasets in MySQL, especially if the data is not evenly distributed across the groups. Therefore, it's recommended to use this method when appropriate to optimize performance.

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