13 th july 2004 Connectivity The term connectivity is used to describe the relationship classification (e.g., one-to-one, one-to-many, and many-to-many). In Figure 4.15 the PROFESSOR teaches CLASS is a one-to-many relationship because each individual professor may teach many classes, but each individual class is taught by one and only one professor. The STUDENT enrolls in CLASS is a many-to-many relationship because each individual student may enroll in many classes and each individual class may have many students enrolled in it. Cardinality Cardinality expresses the specific number of entity occurrences associated with one occurrence of the related entity. Cardinality is usually shown next to an entity on the opposite side of the relationship line. The easiest way to explain it is to use an example. In Figure 4.16 PROFESSOR has a one-to-many (connectivity) relationship to CLASS. The cardinality on the PROFESSOR side of the relationship is (0,3) this means that each individual professor is related to from 0 to 3 classes - in other words he/she teaches 0 to 3
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