A primary key constraint is a unique key with special attributes that make the key the primary access path for the file.
Primary key constraints identify a field or set of fields in a database file whose values must be unique across records in the file. The field must be in ascending order, and can be null-capable. If it is null-capable, a check constraint is implicitly added so that null values cannot be entered in the field. You can define only one primary key constraint for a file.
A primary key constraint can be used as the parent key when adding a referential constraint.