In the Customers Table above, the field (variable) CustomerID is a primary key because it uniquely identifies each record. In the Transaction Table, the field (variable) CustomerID is also present. However, there are multiple records with the same CustomerID values. In the Transaction Table, the field (variable) CustomerID cannot uniquely identify a record by itself; some combination of CustomerID and Date, for example, uniquely identifies each record.

In the Transaction Table, therefore, CustomerID serves as a Foreign Key: a field (variable) that exists to create relationships between two tables, linking to the primary key in another table.