PostgreSQL: Difficulties with DISTINCT ON and ORDER BY leading to unexpected results
I'm a bit lost with I've searched everywhere and can't find a clear answer. I'm relatively new to this, so bear with me... I'm working with a perplexing scenario while using `DISTINCT ON` in PostgreSQL 14. I want to select unique records based on a specific column but keep the most recent entry according to another column. My query looks something like this: ```sql SELECT DISTINCT ON (user_id) user_id, action, created_at FROM user_actions ORDER BY user_id, created_at DESC; ``` The intention here is to get the latest action for each user, but the results are not as expected. Instead of returning the latest action for each `user_id`, I sometimes get older actions. I've also tried adjusting the `ORDER BY` clause without success: ```sql SELECT DISTINCT ON (user_id) user_id, action, created_at FROM user_actions ORDER BY user_id, created_at, action; ``` This seems to affect results when multiple actions have the same timestamp. The query returns inconsistent results depending on the data in the table. I have also checked for duplicates in the `user_id` and `created_at` columns, but no evident duplicates exist. What could be causing this scenario? Is there a best practice for ensuring unique, ordered results in such cases? Any insights or corrections to my approach would be greatly appreciated! My development environment is Linux. What am I doing wrong? This is part of a larger API I'm building. Is there a better approach? My development environment is CentOS. Thanks for your help in advance!