Number of relation instances the cardinality of a

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Number of relation instances The cardinality of a (binary) relation defines the number of relation instances that an entity may participate in [Elmasri and Navathe 2006] . If no cardinalities are annotated for a specific relation type, it is assumed that an arbitrary number of entities (in other words, at least zero entities) may participate in such a relation. Using cardinalities for relations therefore limits the number of instances that are principally possible in an entity- relationship diagram. Example of an Entity-Relationship Diagram The entity-relationship model shown in figure 6-7 shows four entity types (i.e., classes of entities) and three relation types (i.e., classes of relation- ships). The individual entity types possess attributes that describe specific properties of the associated entities. For example, the entity type “traffic jam information” has the attributes “road”, “start”, and “length”, which depict the road on which a traffic jam is currently present, the GPS coor- dinates of the starting point of the jam, and the length of the jam. The rela- tion type “queries” between the entity types “navigation device” and “traffic jam information” means that on the instance level, a relationship between a concrete navigation device and the information on zero or more concrete traffic jams exists. The cardinalities of the entity types with regard to the relation type “queries” means that a concrete navigation device can query information on an arbitrary amount (“N”) of traffic jams. In the other direction, any traffic jam information can be queried by an arbitrary number (“M”) of navigation devices. Figure 6-7 Entity-relationship diagram (data model) according to Chen Ø Ø A
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