A join is performed by comparing two columns. The joined columns hold the same type of information. In the example from the previous chapter, customer.customer_id and salesorder.customer_id hold the same type of number, a customer id number. Customer.customer_id is called a primary key because it is the unique (primary) identifier for the customer table. Salesorder.customer_id is called a foreign key because it holds a key to another (foreign) table.