Wow! We're pretty much done with this part of the course.

Let's summarize what we have learned so far:

1. A single-column primary key is a NOT NULL column, whose value is unique for a given row. For instance:

   CREATE TABLE movie (
     id integer PRIMARY KEY,
     title varchar(64),
     year integer
   );

2. A multi-column primary key is a group of NOT NULL columns, whose values constitute a unique combination for a given row. For instance:

   CREATE TABLE order_item (
     order_id integer,
     sequence integer,
     name varchar(32),
     quantity integer,
     PRIMARY KEY (order_id, sequence)
   );

3. You can add a NOT NULL constraint to a column. A NOT NULL column must have a value other than NULL in every row. For instance:

   CREATE TABLE rpg_game (
     id int PRIMARY KEY,
     name varchar(32) NOT NULL,
     genre varchar(32)
   );

Before we let you go, we've got a short quiz! Are you ready?