Question 15

Question

Question 15

BlackTom AI · Accepted Answer

A foreign key enforces referential integrity: it requires that a value in one table corresponds to an existing primary key value (or unique key) in a related table.

1) Relevant knowledge points and concepts
- Foreign key: a column (or set of columns) in one table that references the primary key or unique key of another table.
- Referential integrity: the database constraint that prevents references to non-existent rows (no “orphan” references).
- Primary key / UNIQUE constraint: ensure uniqueness of rows, which is different from referential integrity.
- Other DB features: denormalization (storing related data together) and encryption are unrelated to foreign key behavior.

2) Step-by-step reasoning
- Suppose you have Customers(id) as the primary key and Orders(customer_id) as a foreign key referencing Customers(id).
- When inserting an order with customer_id = 42, the DBMS checks Customers to confirm id = 42 exists.
- If id = 42 does not exist, the insert is rejected (unless the constraint is temporarily disabled).
- This ensures Orders.customer_id always points to an existing Customers row.

3) Intermediate “calculation” / example steps
- Customers contains ids {1, 2, 3}.
- Attempt to insert Orders with customer_id = 4 → DB rejects because 4 ∉ {1,2,3}.
- Insert Orders with customer_id = 2 → DB accepts because 2 ∈ {1,2,3}.

4) Why the correct answer is correct
- Option c precisely states the role of a foreign key: ensuring a value in one table matches an existing row in a related table (i.e., maintaining referential integrity).

5) Why the other options are incorrect
- a: Describes denormalization; foreign keys do not store all related data in a single table and joins are still common.
- b: Describes primary key or UNIQUE constraint behavior; foreign keys do not by themselves make rows unique and can allow duplicate foreign-key values.
- d: Describes encryption, which is unrelated to referential integrity or foreign keys.

6) Conclusion
The correct answer is c. It correctly describes that a foreign key helps ensure a value in one table matches an existing row in a related table. This matches the provided answer.

Question 15单项选择题

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