advanced patterns When Using COALESCE with Conditional Aggregation in SQL Server 2019
I'm having trouble with I've hit a wall trying to Can someone help me understand I'm working with an unexpected result when using the `COALESCE` function combined with conditional aggregation in SQL Server 2019... I have a table `Sales` with columns `OrderID`, `ProductID`, and `Quantity`. My goal is to get the total quantities for each `ProductID`, but if there are no sales for a given product, I want to return 0 instead of NULL. However, the query I'm using is returning NULL for products without any sales, and I need to figure out why. Here’s the query I wrote: ```sql SELECT ProductID, COALESCE(SUM(Quantity), 0) AS TotalQuantity FROM Sales GROUP BY ProductID; ``` I expected `TotalQuantity` to show 0 for products with no sales, but instead, I get NULL values appearing in the results when certain `ProductID`s do not have any associated sales records. After some debugging, I noticed that the `COALESCE` function seems to be applied after the aggregation. I've also tried using a `LEFT JOIN` with a separate product list, but that didn’t solve the question either. Here’s the modified query: ```sql SELECT p.ProductID, COALESCE(SUM(s.Quantity), 0) AS TotalQuantity FROM Products p LEFT JOIN Sales s ON p.ProductID = s.ProductID GROUP BY p.ProductID; ``` This still gives me NULL for products that do not exist in the `Sales` table. I’m also aware that SQL Server's behavior might differ from other SQL implementations in terms of NULL handling. Can anyone guide to understand why `COALESCE` isn't working as I expect in this scenario? What am I missing? My development environment is Windows. Has anyone else encountered this? How would you solve this? The stack includes Sql and several other technologies.