A one-to-one cardinality ratio means that:

Question

BlackTom AI · Accepted Answer

Each instance in a one-to-one relationship corresponds to a single instance in the related entity. Key concepts: cardinality describes how many instances of one entity relate to instances of another; a one-to-one cardinality means a strict one-to-one mapping (each entity instance maps to exactly one instance on the other side). In database terms this is enforced by unique constraints or by making the foreign key also a primary key, ensuring uniqueness.

Step-by-step reasoning:
1. Define one-to-one: for every instance A in entity X there is exactly one instance B in entity Y, and vice versa. That is a bijective-like constraint in the context of the participating instances.
2. Check the options against this definition:
   - Option 1 states exactly one instance maps to exactly one instance — this matches the definition.
   - Option 2 describes each instance relating to multiple instances on both sides — that implies many-to-many, not one-to-one.
   - Option 3 (“Many instances can link to many others”) also indicates many-to-many cardinality.
   - Option 4 (“Participation is optional on both sides”) refers to optionality (whether a relationship is mandatory or optional), which is a separate property from cardinality; optional participation could occur in a one-to-one relationship but does not define one-to-one.

Why the correct answer is correct:
A genuine one-to-one cardinality enforces uniqueness on both sides so no instance can be associated with more than one counterpart. The phrasing “an instance of one entity is associated with exactly one instance of the other” precisely captures this uniqueness and exclusivity.

Other options are incorrect because they describe many-to-many relationships (options 2 and 3) or discuss participation/optional constraints rather than cardinality (option 4).

Conclusion: The correct choice is “An instance of one entity is associated with exactly one instance of the other,” which matches the provided answer.

A one-to-one cardinality ratio means that:单项选择题

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