A foreign key is a fundamental concept in relational databases used to establish and enforce a link between the data in two tables. It is a column, or a set of columns, in one table (the referencing table) that refers to the primary key of another table (the referenced table). This constraint ensures referential integrity, meaning that a value in the foreign key column must correspond to an existing value in the referenced table's primary key, or be NULL.

A. Super key: A super key is any set of one or more attributes that, taken collectively, uniquely identifies a record in a table. It does not define a relationship between tables.

B. Simple key: This term describes a key that consists of a single column. While a foreign key can be a simple key, this term only describes its structure, not its function of referencing another table.

C. Composite key: This term describes a key that consists of two or more columns to uniquely identify a record. Like a simple key, this describes structure, not the referential function.

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1. Silberschatz, A., Korth, H. F., & Sudarshan, S. (2020). Database System Concepts (7th ed.). McGraw-Hill.

Section 3.7, "Integrity Constraints": "A foreign-key constraint specifies that the values of the foreign key in any tuple of the referencing relation must be a subset of the values of the primary key in some tuple of the referenced relation." (p. 98).

2. Elmasri, R., & Navathe, S. B. (2017). Fundamentals of Database Systems (7th ed.). Pearson.

Chapter 5, Section 5.2.4, "Referential Integrity, and Foreign Keys": "The referential integrity constraint is specified between two relations and is used to maintain the consistency among tuples in the two relations... A foreign key is a set of attributes FK in a relation schema R1 whose values are required to match the values of the primary key PK of some tuple in another relation schema R2." (p. 143).

3. PostgreSQL 16 Documentation. (2023). Chapter 5. Data Definition.

Section 5.4.5, "Foreign Keys": "A foreign key constraint specifies that the values in a column (or a group of columns) must match the values appearing in some row of another table. We say this maintains the referential integrity between two related tables."