At first glance, the 'ON DELETE SET NULL' option in SQL Server 2005 may not seem like a big deal, but once you've seen how it can be used, I think you will be impressed. The basics? If Order.CustomerID has a foreign key to Customer.CustomerID and a customer is deleted, 'ON DELETE SET NULL' causes the CustomerID field in each of that customer's orders to be set to NULL. It allows the "parent" record to be deleted and sets the "child" record's column to NULL. A normal foreign key says Order.CustomerID must match a record in Customer. An 'ON DELETE SET NULL' foreign key says that if Order.CustomerID is not null, then its value must match a record in Customer.