SQL Server 2019: guide with DISTINCT in a Subquery Causing Unexpected Duplicates
I've been struggling with this for a few days now and could really use some help... I'm testing a new approach and I'm learning this framework and I can't seem to get I'm deploying to production and I need some guidance on I'm stuck on something that should probably be simple... I'm working with an unexpected behavior when using a subquery with a `DISTINCT` clause in SQL Server 2019. I have a query that is supposed to return unique customer orders, but instead, I'm seeing duplicates in the result set. Here’s the query I’m working with: ```sql SELECT DISTINCT c.CustomerID, c.CustomerName, (SELECT DISTINCT o.OrderID FROM Orders o WHERE o.CustomerID = c.CustomerID) AS UniqueOrders FROM Customers c; ``` I expected to get a list of customers with only their unique orders, but it seems that the `DISTINCT` in the subquery is not working as intended. When I check the results, some customers appear multiple times with the same `OrderID`. I’ve tried moving the `DISTINCT` clause to the main query, but that only changes the number of duplicates I see, rather than solving the scenario. It might have something to do with how SQL Server handles subqueries and their execution context. I’ve also considered whether there are duplicate entries in the `Orders` table itself that could be affecting this, so I ran: ```sql SELECT OrderID, COUNT(*) FROM Orders GROUP BY OrderID HAVING COUNT(*) > 1; ``` This confirmed that there are indeed duplicates in the `Orders` table. To ensure that I get the unique orders per customer without duplicates in my results, should I be using a `JOIN` instead of a subquery? If so, how can I write that effectively? Additionally, what’s the best practice to handle these duplicates in the `Orders` table if I want to clean them up later? Any help would be greatly appreciated! This is part of a larger web app I'm building. I'm developing on Ubuntu 22.04 with Sql. This is part of a larger application I'm building. I'd love to hear your thoughts on this. Has anyone dealt with something similar? The stack includes Sql and several other technologies. Thanks in advance! Any pointers in the right direction? I'm developing on Linux with Sql. Thanks for any help you can provide!