Sometimes you can't determine whether something you need to model is an entity or a relationship, especially when there are data attributes involved. Some experts in E-R modeling claims there is, in fact, no substantial difference between an entity and a relationship. A Gerund looks like a relationship, but has its own primary key that is not part of the primary key of one of the entities that participate in the relationship. Note how the intersection attribute QuantityShipped is modeled. We previously examined maximum cardinality.
Cardinality needs to be expressed as a range of values.
Maximum cardinality can be 1 or many. This means an occurrence of one entity cannot exist unless there is an occurrence of a related entity. N relationships where the cardinality is mandatory-one. The primary key of the parent entity is used as part of the primary key of the dependent child entity. This situation is also called an identifying relationship .