Having an optional foreign key (ie. sometimes NULL) ?

Hello,

I have the following tables:

- company (eg. ABC Shipping Inc.)
- product (eg. table, chair, pen, pencil)
- client (eg. Joe's Insurance Company)

and they relate to each other as follows:

- a company has clients (company is a foreign key in client)
- a company sells products (company is a foreign key in client)

which is all well and good until I come to one specification in my design
document which says that "some products will only be available to a single
client whereas other products will be available to all clients".

I thought the best way to get around this would be to have the client as a
foreign key in products but for products available to all clients this won't
work.

Can someone advise how I can overcome this?

Thanks, Tom.