SQL Server: Unexpected NULL values when using LEFT JOIN with COALESCE in aggregate functions
Does anyone know how to I've been banging my head against this for hours. Quick question that's been bugging me - I'm working with an scenario where my query returns unexpected NULL values when I use `LEFT JOIN` alongside `COALESCE` in aggregate functions. I'm working with SQL Server 2019, and my goal is to get the total sales amount from the `Sales` table while including customers from the `Customers` table even if they have no corresponding sales record. However, I noticed that when a customer has no sales, the total sales amount is coming back as NULL instead of 0. Here’s a simplified version of my query: ```sql SELECT c.CustomerID, COALESCE(SUM(s.Amount), 0) AS TotalSales FROM Customers c LEFT JOIN Sales s ON c.CustomerID = s.CustomerID GROUP BY c.CustomerID; ``` Despite using `COALESCE`, the `TotalSales` for customers with no entries in the `Sales` table still returns NULL. I verified that there are customers without sales, but I would expect the `COALESCE` to handle this situation. I've also tested the aggregation without `COALESCE`, and it does return NULL for those customers, but I'm not sure why it doesn't seem to be working as intended with `COALESCE` in this case. I've looked into the execution plan and tried to see if there are any filtering issues, but everything appears to be straightforward. Could this behavior be related to how SQL Server handles aggregate functions with NULLs? Any insights or suggestions for resolving this would be greatly appreciated. For context: I'm using Sql on Windows. What's the best practice here? My development environment is Ubuntu. Has anyone else encountered this? This is part of a larger web app I'm building.