That's it! You've mastered everything we've prepared for you so far.

Let's summarize what we learned in this part:

1. A foreign key is a link between two tables. It ensures that a column in table A (which is marked as a foreign key) references a correct value in a column in table B. Here is an example of a foreign key:

   CREATE TABLE position (
     id integer PRIMARY KEY,
     title varchar(255) NOT NULL,
     department_id integer NOT NULL,
     FOREIGN KEY (department_id)
       REFERENCES department (id)
   );

2. A foreign key may consist of multiple columns:

   CREATE TABLE department (
     id integer PRIMARY KEY,
     office_floor integer NOT NULL,
     office_building_name varchar(255) NOT NULL,
     FOREIGN KEY (office_floor, office_building_name)
       REFERENCES office (floor, building_name)
   );

3. You can have more than one foreign key in a single table.

Now, how about a few quick exercises before we let you go?