A primary key is a unique identifier for each record in a table. However, another table can reference that primary key and when it does, we call it a foreign key. This creates a relationship between the two tables.

Multiple records in the referencing table can point to the same foreign key value , so while a primary key must be unique, a foreign key doesn’t have to be. In other words, a foreign key is not unique because it can appear many times, but it always points to a unique primary key in another table.