A NULL is not the same as no data.
NOT NULL constraint restricts a column from having a NULL value. Once NOT NULL constraint is applied to a column, you cannot pass a null value to that column. It enforces a column to contain a proper value. One important point to note about NOT NULL constraint is that it cannot be defined at table level.
CREATE TABLE emp ( e_id INT NOT NULL, lastName VARCHAR(255) NOT NULL, firstName VARCHAR(255), address VARCHAR(255), )