Disjoint rule semantics — judge the statement: “If an entity adheres to the disjoint rule, then an entity instance can be a member of more than one subtype for a given supertype.” Choose whether this is correct or incorrect.

Introduction / Context:
Specialization constraints include disjointness vs. overlap. Under the disjoint rule, a supertype instance may belong to no more than one subtype at a time. Overlapping allows membership in multiple subtypes simultaneously. This question asks whether the disjoint rule permits membership in more than one subtype.

Given Data / Assumptions:

• We consider a single supertype with two or more subtypes.
• Disjointness is an exclusivity constraint, independent of total/partial participation.
• Implementation may enforce the rule via constraints or application logic.

Concept / Approach:
By definition, “disjoint” means mutual exclusivity. If an instance is classified into one subtype, it cannot simultaneously be classified into another subtype of the same supertype. Therefore, the statement that an instance “can be a member of more than one subtype” contradicts the definition of disjointness and is incorrect.

Step-by-Step Solution:

Verification / Alternative check:
In relational mapping, disjointness is often enforced with a single discriminator attribute that can take exactly one value from the set of subtype codes—making multiple memberships impossible.

Why Other Options Are Wrong:

• “Correct” misstates the core meaning of disjointness.
• “Correct only when partial” still wrong; participation does not affect exclusivity.
• Composite keys and DBMS choice do not alter the logical constraint.

Common Pitfalls:
Confusing disjointness with total specialization; using multiple boolean flags that accidentally allow overlapping when disjointness is required; failing to validate constraints in the database.

Final Answer:
Incorrect