We have just said that you can't have two primary keys in one table - but your primary key can consist of more than one column. Take a look at the example:
CREATE TABLE order_item (
PRIMARY KEY (order_id, sequence)
We've got a table which keeps information about items in a given order. We keep the order_id from each item. We also keep the sequence number of the item in the purchase.
order_id isn't enough as the primary key - many items can be ordered together in one purchase and treated as one order. On the other hand, we can't use the
sequence column as the primary key either - the first purchase starts with item no 1, followed by no 2 etc., but the next purchase starts the counting again from 1. In order to obtain unique values for the primary key, we have to take the two columns together.
Primary keys like this - with more than one column - are often called composite primary keys.