Which of the following best describes the purpose of a primary key?

Question

BlackTom AI · Accepted Answer

A primary key uniquely identifies each record (instance) in a table or class, guaranteeing that no two rows share the same key value.

1) Relevant knowledge points and concepts
- Primary key: a column or set of columns whose values uniquely identify each row; enforces entity integrity (no duplicate or null primary key values).
- Foreign key: a column that references a primary key in another table to represent relationships.
- Candidate key: any column(s) that could serve as a primary key; one is chosen as the primary key.
- Multiplicity (in UML): specifies how many instances of one class may relate to instances of another (not a primary-key responsibility).
- Linking tables/junction tables: used to implement many-to-many relationships, often composed of foreign keys.

2) Step-by-step reasoning
- Purpose of a primary key is uniqueness: it must uniquely identify each instance. This prevents duplicate identity and allows consistent retrieval, updates, and deletions.
- While primary keys are referenced by foreign keys, that referencing is a secondary effect; the primary role is identification, not matching foreign keys.
- Linking tables and multiplicities describe relationships and cardinality; they are design constructs separate from the primary key’s identity function.

3) Calculations
- None required for this conceptual question.

4) Why the correct answer is correct
- "Uniquely identifies each instance of the class." directly matches the definition and purpose of a primary key: to provide a unique, non-null identifier for each instance so the database can distinguish records.

5) Why other options are incorrect
- "Allows linking two or more tables.": That is achieved via foreign keys and junction/linking tables, not the primary key itself (though primary keys are referenced).
- "Indicates the minimum multiplicities for an association.": Multiplicity is a UML/ER modeling concept, not the role of a primary key.
- "Matches values of foreign keys in other classes.": Matching is the job of foreign keys referencing primary keys; the primary key is the target, not defined to perform the matching.

6) Conclusion
The correct answer is: "Uniquely identifies each instance of the class." This matches the authoritative answer provided.

Which of the following best describes the purpose of a primary key?单项选择题

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