Use of a supertype/subtype relationship is necessary when which of the following exists?

Question

BlackTom AI · Accepted Answer

Question restatement: Use of a supertype/subtype relationship is necessary when which of the following exists?

Option 1: 'An instance of a subtype participates in a relationship that is the same as the other subtypes.' This suggests the relationship is identical across all subtypes, which means the specialization may not require a distinct subtype relationship to capture unique behavior. In such cases, a flat model without subtypes could suffice, so this option does not justify the need for a supertype/subtype structure.

Option 2: 'Attributes apply to all of the instances of an entity type.' If every instance, including all subtypes, shares the same attributes, there is no necessity to separate into subtypes based on attributes. Subtyping is typically used to model differing attributes or relationships, so this statement undermines the reason for subtyping.

Option 3: 'An instance of a subtype participates in a relationship that is unique to that subtype.' This aligns with the core justification for subtyping: when a particular subtype has relationships (or attributes) that no other subtype shares, a separate subtype structure is needed to accurately capture that specialization and avoid attributing those unique characteristics to the entire supertype.

Option 4: 'No attributes apply to any of the instances of an entity type.' If no attributes differentiate instances, subtyping offers no modeling advantage for attributes or relationships, making subtyping unnecessary. In practice, the lack of attributes across all instances does not motivate a subtype hierarchy.

Analytical takeaway: The key idea behind supertype/subtype modeling is to represent specializations where a subset (a subtype) has distinct relationships or attributes not shared by other subsets. Among the options, only Option 3 expresses a scenario where a relationship is unique to a subtype, which justifies the use of a supertype/subtype relationship. The other options describe conditions that do not require or even contradict the need for subtyping, such as uniform relationships/attributes across all instances or the absence of any differentiating attributes.

Use of a supertype/subtype relationship is necessary when which of the following exists?

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