Final answer:
A foreign key is a field in a database table that creates a relationship between two tables by uniquely identifying a row of another table, ensuring referential integrity within the database.
Step-by-step explanation:
A foreign key is a concept in the realm of databases, particularly in relation to relational databases. It is used to establish a link between two tables. A foreign key in one table is a field (or a collection of fields) that uniquely identifies a row of another table. The purpose of the foreign key is to maintain referential integrity within the database.
How Does a Foreign Key Work?
For example, consider two tables: Orders and Customers. The Customers table has a unique identifier for each customer, typically known as the primary key. This could be something like a customer ID. The Orders table, where each order is recorded, includes a column that contains the customer ID that comes from the Customers table. This column in the Orders table is the foreign key. It creates a relationship between the Orders and Customers tables, allowing for a query to join information from both tables, ensuring that each order is associated with a valid customer.
Foreign keys also enforce referential integrity. If a foreign key exists in one table, any changes to the primary key that it references in another table are restricted. This means that if the primary key that a foreign key references is updated or deleted in its home table, the database system will prevent this change if it would leave the foreign key without a correspondingly valid reference.