Having selected the entity types to include in the database, the data modeler then specifies the relationships
among the entities. For instance, we mentioned previously the advisor/advisee relationship between professors
and students. One of the important decisions to make is whether the relationship will be 1:1 (one-to-one),
1:N (one-to-many), or N:M (many-to-many). These ratios are called cardinality ratios.
In the case of the advisor/advisee relationship, the designer might decide the relationship is 1:N, with
1 advisor advising N students. On the other hand, if the school assigns multiple advisors to each student (for
instance, one for the student??™s major field and one for student life questions), the relationship could be defined
as N:M, multiple advisors for each student, and multiple students for each advisor.
Another pair of decisions related to the cardinality ratio of a relationship is the specification of minimum
cardinalities. Must a student have an advisor? If so, then the minimum cardinality on the professor side of
the advisor/advisee relationship must be 1. If not, then the minimum cardinality on the professor side of the
relationship will be 0; a student entity may exist who is not associated with any advisor.
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