No Null in Primary Key Fields

It case anyone in class missed my comment...

When you say that a set of fields compose the primary key, you are saying that by knowing the values of those fields you can correctly identify the values of the fields in the rest of the record. And since a primary key is a candidate key, then you need all the fields in order to correctly know the values of the rest of the records. Therefore, if you have a null value in one of the primary key fields, then you dont actually need that field to determine the rest of the fields, which means the primary key is not a candidate key (since it can be further reduced).

So if you ever find out that one of your primary key fields is null, then your primary key is not a candidate key and you'll need to change it. Either choose a different candidate key or remove that one field from your primary key and check to see if its a candidate key.