What is a foreign key in relational databases?

• A. A field in a table used to create a relationship to another table.
• B. The primary identifier of a record within the same table.
• C. A constraint that ensures unique values across all tables.
• D. A method for encrypting data at rest.

Answer: (A)

In relational databases, relationships between tables are made with keys. A foreign key is a field (or set of fields) in one table that points to the primary key of another table, creating a link between the two and enforcing referential integrity—the references must match existing records in the other table. This is how you relate related data across tables, and it allows operations like joins to pull together related rows.

The other descriptions don’t fit: the field that uniquely identifies a row within its own table is the primary key; a constraint that ensures unique values across all tables isn’t how foreign keys function (and isn’t accurate across multiple tables); encrypting data at rest is about security, not table relationships.