Scenario: CUSTOMER (CustomerID PK) and ORDER (OrderID PK). One customer may place many orders. Where should the foreign key go?

Question

BlackTom AI · Accepted Answer

In a one-to-many relationship the foreign key belongs on the “many” side, so it should be placed in ORDER referencing CUSTOMER.

Relevant concepts: primary key (PK) uniquely identifies a row (CustomerID on CUSTOMER, OrderID on ORDER). A foreign key (FK) is a column whose values reference a PK in another table to enforce referential integrity. For a 1-to-many relationship (one CUSTOMER → many ORDER), each order must record which customer placed it; therefore the ORDER table holds the FK CustomerID that references CUSTOMER.CustomerID.

Step-by-step reasoning:
1. Identify entities and keys: CUSTOMER(CustomerID PK), ORDER(OrderID PK).
2. Identify cardinality: “One customer may place many orders” = 1 (CUSTOMER) to many (ORDER).
3. Place FK on the many side: Add CustomerID column to ORDER and declare it a foreign key referencing CUSTOMER(CustomerID).
4. Enforce integrity: ORDER.CustomerID NOT NULL (if every order must have a customer) and add FOREIGN KEY (CustomerID) REFERENCES CUSTOMER(CustomerID).

Example SQL:
CREATE TABLE CUSTOMER (CustomerID INT PRIMARY KEY, Name VARCHAR(100));
CREATE TABLE ORDER (OrderID INT PRIMARY KEY, CustomerID INT, FOREIGN KEY (CustomerID) REFERENCES CUSTOMER(CustomerID));

Why the correct answer is correct: Placing the FK in ORDER models that each order is connected to exactly one customer while allowing each customer to have multiple orders. This is the standard relational design for 1-to-many.

Why other options are incorrect:
- In CUSTOMER referencing ORDER: would force each customer to point at a single order (wrong direction) and cannot represent multiple orders per customer.
- In both tables to ensure consistency: redundant keys create update anomalies and are unnecessary for a simple 1-to-many.
- In a separate associative table: associative (junction) tables are used for many-to-many relationships, not needed here.

Final confirmation: The correct placement is “In ORDER referencing CUSTOMER,” which matches the provided answer.

Scenario: CUSTOMER (CustomerID PK) and ORDER (OrderID PK). One customer may place many orders. Where should the foreign key go?单项选择题

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