This is true. In such cases the relationship is said to have "optionality".
The primary key field may NEVER be null. Don't be so stupid!
Third normal form is certainly desirable, however relations which are only in first normal form may be stored in a relational database.
Many to many mappings may be represent - usually by introducing an extra table of "pairs".
This restriction is not general. Some implementations (notably dBase III) require primary key fields to be adjacent.
Which of the following statements concerning relational databases is true?