In a physical data model, what does a foreign key represent?

A foreign key is a column or a set of columns in one table that uniquely identifies a row of another table (or the same table). Its primary purpose is to link the two tables, thereby establishing and enforcing a relationship between them. This mechanism is fundamental to relational database design, ensuring referential integrity by preventing actions that would leave "orphan" records with no corresponding entry in the primary table.

A. Ordering records is a function of the ORDER BY clause in a query, not a structural component like a foreign key.
B. Uniquely identifying each record within its own table is the function of a primary key or a unique key constraint.
D. While a foreign key is an attribute, its specific purpose is to create a relationship, not just to define a data field.

1. Oracle Corporation. (2013). Oracle Database Concepts, 12c Release 1 (12.1). Part No. E41084-01. Chapter 5, "Data Integrity," Section "Referential Integrity Constraints." This section states, "A foreign key... establishes a relationship between a primary or unique key..."

2. Elmasri, R., & Navathe, S. B. (2011). Fundamentals of Database Systems (6th ed.). Addison-Wesley. Chapter 5, "The Relational Data Model and Relational Database Constraints," Section 5.2.4, "Referential Integrity, and Foreign Keys." It defines a foreign key as the mechanism for specifying referential integrity constraints between two relations.