It is set in the pre-planned structure that any column is able to get NULL. In case, you are not in the favor of NULL value then you can prevent it by using constraint. In this way, no NULL value can come in your column.
Here is an example,
(JIM integer NOT NULL,
Last_Name varchar (31) NOT NULL,
Here, JIM and LAST_NAME are the columns which can not use NULL values.
Here is a trial effort for the above statement:
The output will show us error. The reason is that JIM column is NULL which is against the rule of the constraints applied on that column.