PostgreSQL: How to Properly Use DISTINCT ON with ORDER BY for Complex Queries?
I'm sure I'm missing something obvious here, but I'm working on a complex query in PostgreSQL 13.2 where I need to retrieve unique records based on a specific column while also incorporating an ordering based on another column... I thought using `DISTINCT ON` would be the right approach, but I'm working with unexpected results. My current query looks like this: ```sql SELECT DISTINCT ON (customer_id) customer_id, order_date, order_amount FROM orders ORDER BY customer_id, order_date DESC; ``` The goal is to get the latest order for each customer, but when I run the query, I sometimes get older records mixed in with the results, which shouldn't happen. I’ve also tried adjusting the `ORDER BY` clause, but I keep running into the same scenario where it returns incorrect order dates. Additionally, I have an index on `customer_id` and `order_date`, but it doesn't seem to improve performance as I expected it would. Can someone explain how I can ensure that I'm consistently getting the latest order for each customer, and if there are any best practices or caveats I should be aware of when using `DISTINCT ON` in these scenarios? I'm also curious if there's a more efficient way to structure this query if I need to add filtering conditions later on. Any insights or examples would be greatly appreciated! I'm working on a application that needs to handle this. I'm developing on Windows 10 with Sql.