Modality of Relation

Modality Of Relation: The modality defines the nature of relationship as either it is optional or mandatory. If the minimum instance in one entity is zero for an instance of another related entity, then the relationship is called optional. The optional relationship is denoted by small circle “O” on the relationship line.

Similarly if the minimum instance in one entity is one for an instance of another related entity, then the relationship is referred to as mandatory. It is denoted by a small vertical line “│” on the relationship line. If the maximum and minimum cardinalities are both one, it is known as Mandatory one Cardinality in ER diagram.