SQL Server: Unexpected Behavior with LEFT JOIN and WHERE Clause on Non-Nullable Columns
I'm trying to debug I tried several approaches but none seem to work... I'm encountering an unexpected behavior when using a LEFT JOIN in SQL Server. I have two tables, `Orders` and `Customers`. The `Orders` table has a foreign key linking to `Customers`, and I'm trying to retrieve all orders along with customer details. However, when I add a `WHERE` clause to filter out orders from customers in a specific region, it seems to eliminate the non-matching rows from the results, which contradicts my expectation of the LEFT JOIN. Here’s a simplified version of my query: ```sql SELECT o.OrderID, o.OrderDate, c.CustomerName, c.Region FROM Orders o LEFT JOIN Customers c ON o.CustomerID = c.CustomerID WHERE c.Region = 'North' ``` I expected to see all orders, including those where there’s no matching customer (where `c.CustomerName` would be NULL). Instead, I only get orders from customers in the 'North' region, and all unmatched rows from `Orders` are removed. I’ve tried using a different approach by moving the `WHERE` clause into the `ON` condition: ```sql SELECT o.OrderID, o.OrderDate, c.CustomerName, c.Region FROM Orders o LEFT JOIN Customers c ON o.CustomerID = c.CustomerID AND c.Region = 'North' ``` This modification gives me the expected results, showing orders with NULL values for customers not in the 'North' region. However, it feels a bit counterintuitive to me. Why does the behavior differ so significantly between placing the filter in the `WHERE` clause versus the `ON` clause? Is there a best practice for handling such scenarios in SQL Server? I'm using SQL Server 2019 and would appreciate any insights or recommendations on this issue. This issue appeared after updating to Sql latest. I'm open to any suggestions.