PostgreSQL: Difficulty with DISTINCT ON and ORDER BY in Complex Queries
I keep running into I've tried everything I can think of but I'm encountering unexpected results when using `DISTINCT ON` combined with `ORDER BY` in a complex PostgreSQL query. I have a table `orders` that includes a customer ID, order date, and total amount. My goal is to fetch the latest order for each customer based on the order date. However, the results seem to be returning duplicate customer entries, which I didn't expect. Here’s the query I attempted: ```sql SELECT DISTINCT ON (customer_id) customer_id, order_date, total_amount FROM orders ORDER BY customer_id, order_date DESC; ``` When I run this, I get duplicates for some `customer_id`s, which means the `DISTINCT ON` isn't working as intended. I was expecting to see only the latest order for each customer. I've also tried adjusting the `ORDER BY` clause like this: ```sql SELECT DISTINCT ON (customer_id) customer_id, order_date, total_amount FROM orders ORDER BY customer_id, total_amount DESC, order_date DESC; ``` However, this didn't resolve the issue either, and I still see duplicate entries for some customers. I'm using PostgreSQL 13.2, and I’ve verified that there are no typos in the column names. My `orders` table is structured as follows: ```sql CREATE TABLE orders ( order_id SERIAL PRIMARY KEY, customer_id INT NOT NULL, order_date TIMESTAMP NOT NULL, total_amount DECIMAL(10, 2) NOT NULL ); ``` I'd appreciate any insights into why this might be happening and how to properly implement `DISTINCT ON` in this context. Are there any best practices or common pitfalls I should be aware of when using it in more complicated queries? I'm developing on Windows 11 with Sql. This is happening in both development and production on Ubuntu 20.04. Any help would be greatly appreciated!