Connectivity and Cardinality
The connectivity of a relationship describes the mapping of associated entity instances in the relationship. The cardinality of a relationship is the actual number of related occurrences for each of the two entities. In E-R Modeling, we can design data model with three different types of connectivity and Cardinality Relationships.
One instance of an Entity is associated with Zero or One instance of another Entity is called One-To-One Connectivity and Cardinality of Relationship.
An instance, an I.T. Company is constructed with many Building and Workstations. Most of the workstations are allotted to Employees and some workstations are empty that reserved for new Employees.
In general, One-To-One Entities Relationships can merge from two different Entities into one Entity in the E-R Diagram and rarely define two different entities with Primary and Foreign Key Relationships.
When, you focus the diagram from Left to Right side, the diagram representing that Every Employee is allotted with one Workstation and Each Employee must have a workstation by representing the Connectivity and Cardinality of Relationship Mandatory Symbol.
When, you focus the diagram from Right to Left side, the diagram representing that Every Workstation is allocated for an Employee and some Workstations are not allocated to any Employee by representing the Connectivity and Cardinality of Relationship Optional Symbol.
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