In a relational database management system, what is a foreign key?

Question

BlackTom AI · Accepted Answer

A foreign key is a column (or set of columns) in one table that contains values matching the primary key of another table, establishing a relationship between the two tables.

1. Relevant knowledge points and concepts
- Primary key: a column (or set) that uniquely identifies each row in a table and typically has a UNIQUE and NOT NULL constraint.
- Foreign key: a column (or set) in a different table whose values correspond to the primary key values in the referenced table, used to enforce referential integrity.
- Referential integrity: rules that ensure relationships between tables remain consistent (e.g., you cannot reference a non-existent primary key).

2. Step-by-step reasoning process
- Identify what a foreign key does: it links rows between tables by storing identifiers that point to the primary key of another table.
- Example: Customers(CustomerID PK, Name). Orders(OrderID PK, CustomerID FK, Date).
- Orders.CustomerID stores values like 1, 2, 3 that match Customers.CustomerID values, so each order is associated with an existing customer.

3. Intermediate/example steps
- Create Customers rows: (CustomerID=1, Name=Alice), (CustomerID=2, Name=Bob).
- Insert Order: (OrderID=100, CustomerID=1) — valid because CustomerID=1 exists in Customers.
- Attempt Insert Order: (OrderID=101, CustomerID=99) — rejected if referential integrity enforced because 99 is not a Customers.CustomerID.

4. Why the correct answer is correct
- Option (a) precisely describes this relationship: a foreign key is a field in one table that is the primary key in another (or references it). That is the textbook definition.

5. Why other options are incorrect
- (b) Auto number is an implementation detail (an auto-increment type), not the definition of a foreign key.
- (c) A field that must be unique describes a primary key or a UNIQUE constraint, not a foreign key (foreign keys may allow duplicates).
- (d) A foreign key is not the same as a primary key; it references a primary key in another table but does not itself uniquely identify rows in its own table.

6. Conclusion
- The correct answer is (a). This explanation confirms that a foreign key is a field in one table that references the primary key in another table.

In a relational database management system, what is a foreign key?单项选择题

类似问题

Following are the definitions of 2 tables in a DBMS: EMPLOYEE(EmpID,FirstName,LastName,PositionID,Salary) POSITION(PositionID,PositionName,DeptID,EmplID) What is the foreign key in the Table EMPLOYEE?

Question 15

A table in a database can contain more than one foreign key.

The attribute ClientID in the Orders table would be considered

Which of the following correctly describes a foreign key in the entity-relationship (ER) diagram?

If participation of EMPLOYEE in DEPARTMENT is optional, what is the typical implication for the DEPARTMENT_ID foreign key in EMPLOYEE?

All tables in relational databases must have a foreign key.

In relational databases, it is possible to have more than one foreign key in a single table.

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