Foreign Keys and 3NF

Although I've never had a formal education in database design, I've always had an intuitive understanding and appreciation of relational concepts. Studying the first three normal forms has helped me immensely in understand when it is - and is not - a good idea to break columns out in to a separate table.

There is one construct that I have run into in other designer's data models that I don't quite understand. I've always felt intuitively that it is wrong, and it does not appear to be 3NF. To illustrate this suspicious technique, let's take the canonical invoicing example of customer, order, and line item, where a 1 to many relationship exists between customer and order, and between order and line item.

Many designers will add a foreign key to the line item table that references the customer table. This does not seem to be 3NF, since the customer key is completely dependant on the order key. The most common argument for adding this column is to increase the efficiency of a customer - line item join. I've also heard it argued that this column is necessary where it is possible to have a null order record.

What I would like to know is:

Is this actually in third normal form, because foreign keys are special? If it isn't, is it an acceptable denormalization strategy?

I will appreciate any comments.

Thanks,

--
Paul Keister

http://www.dnai.com/~keister