I'm using a Access database and I want to create something like this:

A Table called Account with primary key: CompanyName.
A Table called VisitReport with a primary key that's formed out of three fields: Date, CompanyName, WrittenBy.

The CompanyName must be an existing Account(CompanyName).

I've made a relationship between Account(CompanyName) and ViistReport(CompanyName). For the table VisitReport I've set the three fields mentioned earlier as primary key. So all three fields have that little key symbol infront of them.

Now the filed WrittenBy could be an empty field, but the Access database won't allow it to be empty because it's set as a primary key.

What I'm trying to do is use three fields together as the primary key and allow it if Writtenby is empty.

How to fix this problem?