ER Model and Business Rules Questions and Answers

Answer: B

Correct answer: When a subtype entity inherits values of the supertype attribute

Inheritance runs downward. Attributes common to every subtype are held once at the supertype and are inherited by each subtype instance, so an instance of the subtype carries both the general attributes and its own particular ones. The relationships of the supertype are inherited on the same terms.

Answer: A

Correct answer: True

With disjoint subtypes a single-valued discriminator suffices, since each instance belongs to exactly one. With overlapping subtypes an instance may belong to several, so the discriminator becomes a composite of boolean attributes, one for each subtype, each answering whether this instance is of that kind.

Answer: A

Correct answer: True

Generalisation runs upward: several specific entity types are examined, their common attributes and relationships are abstracted, and a supertype is created to hold them. Specialisation runs the other way, from one general type down to subtypes - the two processes arriving at the same structure from opposite directions.

Answer: A

Correct answer: Disjoint with total specialization

Two rules are being stated at once. Must be a member makes the specialisation total, so no supertype instance sits outside the subtypes. Only one makes it disjoint, so the subtypes do not overlap. Each rule is independent of the other, which is why all four combinations of them exist.

Answer: D

Correct answer: All of the above

The naming conventions for a subtype are those for any entity: a singular noun, because the name denotes one instance; concise, because it will be read many times; and meaningful within the organisation, since a name borrowed from a textbook rather than from the business will not be recognised by the people who must validate the model.

Answer: B

Correct answer: It may include restrictions.

A business rule constrains what the organisation permits, so restrictions are exactly what it expresses. The other options are the opposite of good practice - rules should be simple enough to be understood by the people who own them, and stating them so that they can be enforced in code is the point of capturing them at all.

Answer: B

Correct answer: False

The supertype and subtype line carries no cardinality, because the relationship is not an association between two things - it is an is-a relationship, in which the subtype instance is the supertype instance. What the line does carry is the completeness and disjointness rules.

Answer: A

Correct answer: An attribute of the supertype whose values determine the subtype

The discriminator sits on the supertype because the question it answers - which kind of thing is this - must be answerable before the subtype is known. Its value directs each instance to its subtype: an employee type code of H, S or C sending the instance to hourly, salaried or consultant.

Answer: C

Correct answer: Each subtype has only one supertype.

A hierarchy is a tree: every subtype has exactly one immediate parent, so attributes are inherited along a single path and there is no ambiguity about where a value comes from. Where a subtype genuinely has two parents the structure is a lattice rather than a hierarchy, and multiple inheritance must be dealt with explicitly.

Answer: A

Correct answer: True

The double line indicates total specialisation, meaning every supertype instance must belong to some subtype. A single line indicates partial specialisation, where an instance may belong to none. The convention makes the completeness rule readable at a glance.