ITPEC FE Morning October 2021 Question 27

Which of the following is an appropriate explanation of an E-R diagram?

a)A relationship is expressed by describing the related entity name in the entity type.b)The relationship between entity types is expressed by an arrow from the referencing side in the direction of the referenced side.c)There are no attributes in the entity type, but the relationship type has attributes.d)There are several kinds of relationships between entity types such as one-to-many or many-to-many.

Explanation

Relationships between entity types can be one-to-one, one-to-many or many-to-many — an E-R diagram shows the cardinality of each relationship.

An E-R diagram has entity types (rectangles), relationship types (a diamond or a line between them) and attributes. Each relationship is marked with its cardinality; many-to-many can be drawn directly in the E-R model and is only resolved into an extra table when the relational tables are designed.

Answer (d)

Why not others:

•(a) — writing the related entity's name inside an entity type is how a relational table stores a foreign key; in an E-R diagram the relationship is its own element
•(b) — an arrow from the referencing to the referenced side is the reference direction of a foreign key, not how an E-R diagram expresses a relationship
•(c) — entity types do have attributes; a relationship type may have some as well, but that does not take them away from entities

Key rule: The E-R model is conceptual: entities, relationships with cardinalities, and attributes; foreign keys and reference arrows belong to the relational design that comes after it.

Databases · AI-written, may contain errors

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