Consider an association between a Customers class and a Sale class in a Unified Modeling Language (UML) Class diagram. The multiplicities next to the Customers class are 1..1 and the multiplicities next to the Sale class are 0..*. Which of the following is the best way to implement that association in your database?

Question

BlackTom AI · Accepted Answer

A 1..1 (Customers) to 0..* (Sale) multiplicity describes a one-to-many relationship: one Customer can have zero or many Sales, and each Sale must be associated with exactly one Customer.

1) Relevant knowledge points and concepts
- UML multiplicities: the number at the opposite end of an association tells how many instances of that class can be linked. 1..1 means exactly one; 0..* means zero or many.
- Relational mapping of associations: a one-to-many relationship is implemented by placing the primary key (PK) of the “one” side as a foreign key (FK) in the “many” side table.
- Referential integrity and nullability: if the multiplicity on the “one” side for each instance of the “many” side is 1..1, the FK in the many-side table should be NOT NULL.

2) Step-by-step reasoning
- Read multiplicities: next to Customers is 1..1 (for each Sale, the Customer end must be exactly one). Next to Sale is 0..* (for each Customer, there may be zero or many Sale instances).
- This is one Customer → many Sales (one-to-many).
- In a relational schema, the many side (Sales) must carry the FK referencing the one side (Customers).

3) Intermediate schema example (illustrates steps)
- Customers(CustomerID PK, Name, ...)
- Sales(SaleID PK, CustomerID FK NOT NULL, Date, Amount, ...)
- FK constraint: FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID)

4) Why the correct answer is correct
- Putting Customers’ PK into Sales models that every Sale points to exactly one Customer, enforces referential integrity, and allows a Customer to have multiple Sales.

5) Why other options are incorrect
- Posting Sales’ PK as FK in Customers would require either multiple FK columns or a repeating structure to represent many sales per customer — impractical and incorrect for one-to-many.
- Creating a linking table is appropriate for many-to-many relationships, not for one-to-many.
- “None of these” is incorrect because a direct FK in Sales is the correct implementation.

6) Conclusion
- Therefore, the correct implementation is: Post the primary key of Customers as a foreign key in Sales. This matches the provided answer.

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