SQL Server 2019: Troubleshooting inconsistent results with LEFT JOIN and GROUP BY
I'm working on a project and hit a roadblock. I'm encountering an issue with a query in SQL Server 2019 where I'm trying to get a count of orders per customer, but the results seem inconsistent when I use a `LEFT JOIN` with `GROUP BY`. The goal is to list all customers even if they haven't placed any orders, and to show `0` for the count of orders in such cases. However, I noticed that for some customers, instead of `0`, I'm getting `NULL` or even unexpected counts. Here's the query I'm using: ```sql SELECT c.CustomerID, c.CustomerName, COUNT(o.OrderID) AS OrderCount FROM Customers c LEFT JOIN Orders o ON c.CustomerID = o.CustomerID GROUP BY c.CustomerID, c.CustomerName; ``` I expect to see a count of `0` for customers without orders, but some customers are returning `NULL` in the `OrderCount` column. I haven't applied any filters in the `WHERE` clause, so it's puzzling why I'm seeing these `NULL` values. I've tried using `COALESCE` to ensure that I get `0` instead of `NULL`: ```sql SELECT c.CustomerID, c.CustomerName, COALESCE(COUNT(o.OrderID), 0) AS OrderCount FROM Customers c LEFT JOIN Orders o ON c.CustomerID = o.CustomerID GROUP BY c.CustomerID, c.CustomerName; ``` However, this still returns `NULL` for the `OrderCount` in some rows. Also, I validated that there are indeed customers in the `Customers` table who have no matching entries in the `Orders` table. Could this be related to how SQL Server processes the `LEFT JOIN` and the `GROUP BY`? Is there a better way to achieve the desired output? Any insights would be greatly appreciated! Any ideas what could be causing this?