Okay, it's time to wrap things up! Here's what we learned:

1. To add a NOT NULL constraint when defining a table, use:

   CREATE TABLE loan_application (
     ...
     amount decimal(10, 2) NOT NULL,
     ...
   );

2. By default, the columns accept NULLs. You can emphasize this by putting the NULL keyword after column definition:

   description varchar(255) NULL

   Adding the NULL keyword does not change the column definition, it just emphasizes the default behavior.
3. To add a NOT NULL constraint to an existing table, use:

   ALTER TABLE loan_application
   ALTER COLUMN loan_period_months SET NOT NULL;

4. To delete a NOT NULL constraint from an existing table, use:

   ALTER TABLE loan_application
   ALTER COLUMN loan_period_months DROP NOT NULL;

Ready for the pop quiz?