SQL Server: Unexpected Results with LEFT JOIN and GROUP BY on Nullable Columns
I've encountered a strange issue with I'm performance testing and I'm trying to implement I'm working with unexpected behavior when trying to aggregate data using a `LEFT JOIN` in SQL Server 2019, particularly when dealing with nullable columns. I have two tables, `Orders` and `Customers`, where `Customers.CustomerID` can be null for some records. My goal is to get a count of orders per customer, including customers with no orders. Here’s the query I’m using: ```sql SELECT c.CustomerID, COUNT(o.OrderID) AS OrderCount FROM Customers c LEFT JOIN Orders o ON c.CustomerID = o.CustomerID GROUP BY c.CustomerID ORDER BY c.CustomerID; ``` I expect to see all customers in the result set, with `OrderCount` showing `0` for customers who haven't placed any orders. However, I'm getting a row with `NULL` for `CustomerID` and `OrderCount` as `0`, which I did not anticipate. After some troubleshooting, I tried adding a `WHERE` clause to filter out null values, but this eliminated all customers without orders from the result set. Also, I verified that there are customers in the database that have no corresponding orders. My `Orders` table looks like this: ```sql CREATE TABLE Orders ( OrderID INT PRIMARY KEY, CustomerID INT NULL ); ``` And the `Customers` table is defined as: ```sql CREATE TABLE Customers ( CustomerID INT PRIMARY KEY ); ``` I'm confused about why I'm seeing this `NULL` row in the output and how to correctly handle this scenario while still keeping customers without orders in the results. Any insights on what might be going wrong or how to resolve this would be greatly appreciated! I'm on Debian using the latest version of Sql. Hoping someone can shed some light on this. This is happening in both development and production on Windows 11. Am I approaching this the right way? For reference, this is a production REST API. I appreciate any insights!