can a foreign key contain null values in database?

Question

it is written there in some circumstances it can be null u can see this in http://msdn2.microsoft.com/en-us/library/ms175464.aspx
and http://dev.mysql.com/doc/refman/5.0/en/innodb-foreign-key-constraints.html

Answer 1

Certainly, as long as the FK is not a primary key on another table.

Answer 2

Yes. primary key doesn't allow null value.