SQL Server query with correlated subquery returning unexpected NULL values
I'm maintaining legacy code that I'm getting frustrated with I'm building a feature where I'm trying to implement I am encountering an issue with a query that uses a correlated subquery in SQL Server 2019..... The goal is to retrieve a list of products along with their maximum order quantity from a related Orders table. However, I'm seeing unexpected NULL values for the maximum order quantity where I anticipate valid numbers. Here’s the relevant part of my query: ```sql SELECT p.ProductID, p.ProductName, (SELECT MAX(o.OrderQuantity) FROM Orders o WHERE o.ProductID = p.ProductID) AS MaxOrderQuantity FROM Products p; ``` The `Products` table has a list of products, and the `Orders` table records each order's quantities linked by `ProductID`. When I execute this query, I notice that for some products, the `MaxOrderQuantity` returns NULL, even though there are corresponding entries in the `Orders` table. I initially thought it could be related to products that have no orders at all, but I also checked for products that should have valid maximum quantities. I’ve tried adding an `INNER JOIN` with the `Orders` table instead, but that filtered out the products entirely, which is not the desired behavior: ```sql SELECT p.ProductID, p.ProductName, o.MaxOrderQuantity FROM Products p LEFT JOIN ( SELECT ProductID, MAX(OrderQuantity) AS MaxOrderQuantity FROM Orders GROUP BY ProductID ) o ON p.ProductID = o.ProductID; ``` This modification eliminates the NULL issue but returns only the products that have orders, which does not meet my requirement to list all products regardless of whether they have orders or not. Is there a way to ensure I get the maximum order quantity for all products, while still returning NULL for those that have no orders? Any insights or alternative approaches would be greatly appreciated! I'm using Sql latest in this project. My team is using Sql for this web app. I'd really appreciate any guidance on this. Has anyone dealt with something similar?