Introduction to Cardinality and Modality

Elements of data modelling such as data objects attributes and relationships  provide the basis for understanding the information  domain such  as what  are the composite information s, its properties and how are they interrelated but  such information is not sufficient. For example  we can say that where are two data objects A and B  with  attributes a₁,a₂,a₃..............a_n and b₁.b₂,b₃...........b _m respectively. They are related to each other by relation R as shown in fig 4.3

This information is not sufficient. We must also know how many elements of A are related to how many elements of B by relation R. This concept is called cardinality. There may be some cases when a defined relationship is not mandatory. This concept is called as modality.

Cardinality: Cardinality is the specification of number of occurrences of one object that can be related to number of occurrences of another object. It is usually expressed as either one or many. It generally defines the maximum number of objects that can participate in a relationship. On the basis of cardinality a relationship can be one to one (1:1) one to one (1: M) many to many (M-M).

Modality: Modality of a relationship is represented by 1 if it is mandatory 0 if it is not mandatory.

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