Third normal form is certainly desirable, however relations which are only in first normal form may be stored in a relational database.
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?
A foreign key field may be null.
A primary key field may be null
All relations must be in at least third normal form
Many to many mappings cannot be represented
The primary key fields of a relation must be adjacent
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