>>>I have a table that contains two classifications of items: products & services.
>>>When it is a 'service' I need to be sure that a Header record exists for it.
>>>To that end, I have a FOREIGN KEY on Item.HeaderID referencing Header.HeaderID.
>>>
>>>How can exclude the products, which have a NULL value in Item.HeaderID, from this FOREIGN KEY CONSTRAINT?
>>>
>>>Thanks in advance.
>
>>What is your database? SQL Server or VFP native tables?
>
>SQL-Server 2005 SP2

Live NULL in the Item.HeaderID columnn for products. You can also add constraint that wouldn't allow to change it to anything else for products.