Cardinality and Modality

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Cardinality and Modality

Cardinality and Modality work together to define the relationship.

Cardinality indicates the maximum number of times an instance in one entity can be associated with instances in the related entity.

Modality indicates the minimum number of times an instance in one entity can be associated with an instance in the related entity.

Thus, Modality is also called participation because it denotes whether or not an instance of an entity MUST participate in the relationship.

Cardinality and Modality are both shown on the relationship line by symbols. We will go over each of the symbols and how to interpret them.

Cardinality

Cardinality indicates the maximum number of times an instance of one entity can be associated with instances in the related entity. Cardinality can have the values of one or many, no more detail than that. It is either one or more than one. On the relationship line, the cardinality is the closest to the entity box. The cardinality symbol in the diagram on the slide is in the red circle. Cardinality is indicated at BOTH ends of the relationship line, so there is a left to right cardinality and a right to left cardinality.

Modality

Modality indicates the minimum number of times an instance in one entity can be associated with an instance in the related entity. Modality can have the values of zero or one, two or three are not allowed. The modality symbol is located next to the cardinality symbol, on the inside, i.e., NOT next to the entity box. A modality of one is denoted by a straight vertical line and a modality of zero is denoted by a circle. Like cardinality, modality is indicated at both ends of the relationship.
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