Describe the relationship between Enrolls and COURSE

Question

BlackTom AI · Accepted Answer

Enrolls includes cnnumber as part of its composite primary key and that cnnumber is a foreign key referencing Course.cnnumber, so each Enrolls row must reference exactly one Course.

1) Relevant concepts
- Primary key (PK) attributes are NOT NULL and uniquely identify a row.
- A foreign key (FK) references a primary key in another table to enforce referential integrity.
- Cardinality: “1 and only 1” means the FK is mandatory (cannot be null) and points to exactly one parent row.

2) Step‑by‑step reasoning
- Inspect Enrolls: its PK is composed of suniqueID and cnnumber. That means cnnumber is part of the PK.
- Because cnnumber is part of the PK it cannot be null (PK attributes are NOT NULL).
- The cnnumber attribute in Enrolls is a foreign key that references Course.cnnumber (the Course table primary key).
- A non‑null foreign key that references Course implies every Enrolls row must be associated with one, and exactly one, Course row.

3) Intermediate logical steps (explicit)
- Step A: cnnumber ∈ PK(Enrolls) ⇒ cnnumber NOT NULL.
- Step B: cnnumber is FK → Course.cnnumber ⇒ enforces a reference to a Course row.
- Step C: NOT NULL FK referencing Course ⇒ each Enrolls record maps to one and only one Course.

4) Why this answer is correct
Because Enrolls stores the course identifier as part of its primary key and that identifier is a required foreign key to Course, there is no possibility of an Enrolls record existing without exactly one Course. Thus from the Enrolls side the cardinality is “1 and only 1” Course.

5) Why other options are incorrect (brief)
- Any option implying 0..1 is wrong because cnnumber cannot be null (it’s part of the PK).
- Options claiming Enrolls associates with many Courses are wrong because a single Enrolls row holds a single cnnumber value (it cannot reference multiple courses).
- Many-to-many or “no relationship” options misunderstand that Enrolls is a join/association table: it links to Course via a mandatory FK.

Conclusion: therefore the correct description is “1 Enrolls is associated with 1 and only 1 Course,” matching the provided answer.

Describe the relationship between Enrolls and COURSE单项选择题

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