mysql not null & allow null difference

Question

I am a mysql beginner, maybe it is a silly question, so don't laugh at me :) I am currently reading a book and confused about the following sentence:

NOT NULL means that all the rows in the table must have a value in the attribute, if it isn't specified, the field can be blank (NULL)

So, even you set a column to not null, you don't have to specify a value to it? Then it become blank(null)? Then what is the difference between not null and allow null? Is the book wrong?

Answer 1

The "if it isn't specified" refers to "NOT NULL" - not the value. So it would have been clearer as:

NOT NULL means that all the rows in the table must have a value in the attribute. If NOT NULL isn't specified, the field can be blank (NULL)