MySQL 8.0: Unexpected Results with DISTINCT and ORDER BY in Subqueries
I'm working on a project and hit a roadblock. I'm working with an scenario in MySQL 8.0 where using `DISTINCT` in a subquery along with `ORDER BY` seems to yield inconsistent results. I have a table named `orders` with columns `id`, `customer_id`, `amount`, and `order_date`. I want to retrieve unique `customer_id`s sorted by the total `amount` they spent. Here’s the query I wrote: ```sql SELECT DISTINCT customer_id, (SELECT SUM(amount) FROM orders o WHERE o.customer_id = o2.customer_id) AS total_spent FROM orders o2 ORDER BY total_spent DESC; ``` However, this returns unexpected results. The output does not seem to reflect the correct totals when I check them manually. For instance, one of the customers with ID `3` should appear at the top based on their total spending, but it’s not. I also tried using a CTE to simplify the logic like this: ```sql WITH CustomerTotals AS ( SELECT customer_id, SUM(amount) AS total_spent FROM orders GROUP BY customer_id ) SELECT DISTINCT customer_id, total_spent FROM CustomerTotals ORDER BY total_spent DESC; ``` This version seems to work better, but I’m confused about why the first query is failing. Is there a specific reason why `DISTINCT` would not work as expected in the presence of an `ORDER BY` clause in a subquery? Additionally, does anyone have recommendations on best practices for achieving this kind of result efficiently? Any insights would be helpful! I'm working on a application that needs to handle this.