PostgreSQL: scenarios with Using DISTINCT ON with ORDER BY on Multiple Columns
I'm collaborating on a project where I've spent hours debugging this and I'm working on a project and hit a roadblock... I'm running into an scenario while trying to use `DISTINCT ON` combined with `ORDER BY` in my PostgreSQL query. I want to fetch the latest record for each unique `user_id` based on the `created_at` timestamp, but I'm getting unexpected results. Here's my query: ```sql SELECT DISTINCT ON (user_id) user_id, created_at, score FROM user_scores ORDER BY user_id, created_at DESC; ``` I expected this to return only the latest `created_at` score for each `user_id`, but it seems that I'm getting multiple entries for some users that have the same `created_at` timestamp, and I need to figure out why. For example, if `user_id` 1 has two scores with the same `created_at` timestamp, I want to select only one of them, but it seems like I'm not controlling which one is returned. I'm using PostgreSQL version 13.2. I've tried adding more columns to the `ORDER BY` clause, thinking it would help, but it hasn't worked as expected: ```sql ORDER BY user_id, created_at DESC, score DESC; ``` However, that still returns multiple entries when the `created_at` timestamps match. How can I ensure that I only get one row per `user_id` when there are ties in the `created_at`? Is there a better approach to achieve what I'm trying to do? Any insights would be appreciated! This is part of a larger application I'm building. The project is a application built with Sql. Am I missing something obvious? This is for a REST API running on macOS. Any help would be greatly appreciated!