why you add a not null in the sales table besides primary key, you can just use primary key because it doesn't accept not null values.

Hi Houda! Thanks for reaching out. Your observation is not incorrect! The column is indeed a primary key in this table. We have not created it as such for now because we are doing that in the upcoming lectures. As you may check the syllabus, we have not yet covered the concept of primary keys so we didn't want to write something that is not yet being thought. Hope this helps. Best, Tsvetelin