Likewise, must every professor be an advisor? If so, then the minimum cardinality on the student side
of the advisor/advisee relationship must be 1. If not, then the minimum cardinality on the student side will be
0; a professor entity can exist with no associated advisees.
Other relationships might be 1:1. Imagine an entity type called Parking Permit, and that the policy is to allow
each student one and only one parking permit. The relationship between student and parking permit could be called
???parks/permit-to-park,??? and the relationship is 1:1. The minimum cardinality on the student side would probably be
1, since otherwise it would mean the database tracks parking permits that are not issued to anyone. The minimum
cardinality on the parking permit side would probably be 0, since some students probably will not have cars to park.
A many-to-many, N:M, relationship would exist between students and courses. We could call this relationship
???takes/is-taken-by.??? Each student will take many courses, and many students will take each course. The
minimum cardinality for both sides of the relationship will be 1, because each student will certainly take some
courses and - each course will be attended by some students.
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