Question

Discuss and define the following: cluster entity type weak relationship type composite relationship type

Answer 1

1. An entity cluster is a “virtual” entity type used to represent multiple entities and relationships in the ERD. An entity cluster is formed by combining multiple interrelated entities into a single abstract entity object. An entity cluster is considered “virtual” or “abstract” in the sense that it is not actually an entity in the final ERD. Instead, it is a temporary entity used to represent multiple entities and relationships, with the purpose of simplifying the ERD and thus enhancing its readability.When using entity clusters, the key attributes of the combined entities are no longer available. Without the key attributes, primary key inheritance rules change. In turn, the change in the inheritance rules can have undesirable consequences, such as changes in relationships from identifying to non-identifying or vice versa and the loss of foreign key attributes from some entities. To eliminate those problems, the general rule is to avoid the display of attributes when entity clusters are used.
2. An Identifying Relationship is a relationship between a strong and a weak entity type, where the key of the strong entity type is required to uniquely identify instances of the weak entity type..In entity relationship diagrams (ER diagrams), a weak entity set is indicated by a bold (or double-lined) rectangle (the entity) connected by a bold (or double-lined) type arrow to a bold (or double-lined) diamond (the relationship). This type of relationship is called an identifying relationship and it is represented by an oval entity rather than a square entity for base tables. An identifying relationship is one where the primary key is populated to the child weak entity as a primary key in that entity.
3. A composite entity is also known as a “bridge” entity. This “bridge” is used to handle the many-to-many relationships that the traditional entity could not handle. This entity lies between the two entities that are of interest and this composite entity shares the primary keys from both the connecting tables. This composite entity is also known as a “gerund” because it has the characteristics of an entity and a relationship.