What is cardinality?

What is cardinality?

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Cardinality indicates the number of instances of the two entities that are allowed to engage in a given instance of a relationship. Following are the three types of cardinality:

• One-to-one: In a one-to-one relationship, every instance of the first entity has only one instance of the second entity. For example, in a company, every department such as Sales has only one Senior Manager, and all the other employees in the Sales department work under him. This relationship between the Sales department and the Senior Manager is one-to-one.



• One-to-many: In a one-to-many relationship, every instance of the first entity can have more than one instance of the second entity, but for the instance of the second entity, there is exactly one instance of the first entity. For example, the relationship between Department and Employee is one-to-many. There are many employees working in a department (e.g. Sales).



• Many-to-many: In a many-to-many relationship, every instance of the first entity can have multiple instances of the second entity, and every instance of the second entity can also have more than one instance of the first entity. For example, a customer can purchase more than one product, and a product can also be sold to several customers.

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