SQL Server: Getting Incorrect Row Count with COUNT() in a Filtered Query with a LEFT JOIN
I'm having a hard time understanding I am working with an scenario with a SQL Server 2019 query where I am trying to get a count of records from one table based on certain criteria from another table using a LEFT JOIN. The expected row count does not match what I am getting. Here’s the situation: I have a `Customers` table and an `Orders` table. I want to count the number of customers who have placed orders in the last month but still want to include those who haven't placed any orders. The query I wrote is as follows: ```sql SELECT COUNT(c.CustomerID) AS CustomerCount FROM Customers c LEFT JOIN Orders o ON c.CustomerID = o.CustomerID WHERE o.OrderDate >= DATEADD(MONTH, -1, GETDATE()) OR o.OrderDate IS NULL; ``` However, this returns a count of all customers instead of just those who either have made an order in the last month or have no orders at all. I also tried modifying the query to use a subquery to filter orders: ```sql SELECT COUNT(c.CustomerID) AS CustomerCount FROM Customers c WHERE c.CustomerID IN ( SELECT o.CustomerID FROM Orders o WHERE o.OrderDate >= DATEADD(MONTH, -1, GETDATE()) ) OR NOT EXISTS ( SELECT 1 FROM Orders o WHERE o.CustomerID = c.CustomerID ); ``` Still, I am not getting the correct count. The result is showing a higher count than expected. I’ve verified that the `OrderDate` field contains valid dates, and the customer data is accurate. I am particularly confused about the `COUNT()` function's behavior with NULL values in this context. What am I doing wrong? Is there a more effective way to structure this query to achieve the desired results? Any insights would be appreciated! This is my first time working with Sql 3.11. Any examples would be super helpful.