A supertype can only have only one subtype.
Answer: B
Correct answer: False
A supertype ordinarily has several subtypes, since the point of the construct is to distinguish kinds. A single subtype is possible but unusual and generally means the modeller has found an exception rather than a genuine classification.
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There is cardinality on an EER between the supertype and subtype.
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.
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A subtype discriminator of the supertype is an attribute whose values determine the target subtype(s).
Answer: A
Correct answer: True
This is the definition. The discriminator is an attribute of the supertype whose value tells which subtype an instance belongs to, so it must be present on every instance and must be examined before the subtype is known.
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Overlapping and disjoint subtypes have different approaches on how the subtype discriminator is applied.
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.
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An entity instance of a subtype represents the same entity instance of a supertype.
Answer: A
Correct answer: True
The supertype and the subtype are two views of one thing, not two things. An instance of the subtype is the corresponding supertype instance, which is what makes attribute inheritance meaningful and why the relationship carries no cardinality.
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A supertype is an entity type that shares common attributes or relationships distinct from other subgroupings.
Answer: B
Correct answer: False
That is the definition of a subtype - a subgrouping sharing attributes or relationships distinct from the other subgroupings. A supertype is the general entity type whose attributes and relationships are shared by all of them. The statement has given one definition under the other name.
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An ERD that has had the entities clustered will display more entities.
Answer: B
Correct answer: False
Clustering groups related entities into a single higher-level object, so the diagram shows fewer symbols. The point is to make a large model readable at a summary level, with the detail visible on decomposing a cluster rather than all at once.
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Total specialization is represented by double lines between the supertype and subtype on an EER.
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.
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Generalization is the process of defining more general entity types from a set of more specialized entity types.
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.
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It is appropriate to use a supertype/subtype relationship when there are attributes that apply to some of the instances of an entity type.
Answer: A
Correct answer: True
This is one of the two conditions justifying the construct. If an attribute applies to only some instances, holding it on the single entity type means most rows carry a null, and the null has no meaning - it is not unknown but inapplicable. A subtype gives it a proper home.
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