Referential integrity means:

Question

BlackTom AI · Accepted Answer

Referential integrity requires that a foreign key value either matches an existing primary key value in the referenced table or is null.

1) Relevant concepts:
- Primary key (PK): a column or set of columns that uniquely identifies each row in a table.
- Foreign key (FK): a column or set of columns in one table whose values refer to primary key values in another (or the same) table.
- Referential integrity constraint: a rule enforced by the DBMS ensuring that FK values correspond to existing PK values (or are null), preserving valid links between tables. Databases can also support cascading actions (ON DELETE/UPDATE CASCADE) and deferred constraint checking, but the core rule remains FK must reference existing PK or be null.

2) Step-by-step reasoning:
- Suppose Table Departments has PK dept_id with values {1, 2, 3}.
- Employees has column dept_id declared as FK referencing Departments(dept_id).
- If you insert Employees.dept_id = 2, DBMS checks: is 2 present in Departments.dept_id? Yes → allowed.
- If you insert Employees.dept_id = 5, DBMS checks: is 5 present? No → constraint violation; insertion rejected unless the FK is nullable and value is set to NULL.
- If Employees.dept_id = NULL and FK allows NULLs → allowed (NULL represents “no current department” and isn’t required to match a PK).

3) Intermediate steps/examples shown above illustrate enforcement — existence check against PK set.

4) Why this answer is correct:
The referential integrity rule is precisely the existence requirement described: FKs point to existing PK values (or are NULL). This prevents references to non-existent rows and maintains consistency across related tables.

5) Why the other options are incorrect:
- "Foreign keys can point to non-existent rows..." contradicts the integrity rule — allowing dangling references breaks relational consistency and is not referential integrity.
- "Composite PK/FK pairs are mandatory..." is false: composite keys are used when needed, but not mandatory for all relations.
- "Any relationship must use a primary key and a foreign key." Relationships can be modeled in various ways (e.g., associative tables, application-level links), and the statement is too absolute; referential integrity specifically constrains FKs, not every conceivable relationship representation.

Conclusion: The correct answer is "Any foreign key value must match an existing primary key value or be null." This matches the provided answer.

Referential integrity means:单项选择题

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