PostgreSQL: Incorrect Results When Using ROW_NUMBER() with PARTITION BY in a Subquery
I'm prototyping a solution and I'm working with issues with a query that utilizes the `ROW_NUMBER()` function along with `PARTITION BY` in PostgreSQL 14... My intent is to retrieve the latest entry for each user based on a timestamp, but I'm getting unexpected results. Here’s the query I'm using: ```sql SELECT user_id, event, created_at FROM ( SELECT user_id, event, created_at, ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn FROM user_events ) AS subquery WHERE rn = 1; ``` I'm expecting to get one row per `user_id` with the latest `created_at` timestamp, but instead, I’m getting multiple rows for some users. I checked the data in the `user_events` table and confirmed that there are indeed unique latest timestamps for each user. The `created_at` column is of type `TIMESTAMP WITH TIME ZONE`. When I run the inner query alone, it returns the expected results: ```sql SELECT user_id, event, created_at, ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn FROM user_events; ``` However, when I wrap it in a subquery, I end up with duplicates in the final output. I’m not sure if this is an scenario with how PostgreSQL handles the subquery or if I’m missing something in my logic. I've also tried using `DISTINCT` in the outer query like this: ```sql SELECT DISTINCT user_id, event, created_at FROM ( SELECT user_id, event, created_at, ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY created_at DESC) AS rn FROM user_events ) AS subquery WHERE rn = 1; ``` This also doesn't resolve the duplication question. It would be great if someone could guide to understand what's going wrong here or suggest an alternative way to achieve the same result. Thanks! This is part of a larger REST API I'm building. Any feedback is welcome!