In database theory, NULL represents unknown or missing information. NULL is not the same as an empty string or number of zeros. Courtesy to this constraint, any row in the column to which this constraint is added cannot be left blank. A value must be entered.
CREATE TABLE table_name( column_name data_type NOT NULL , ....);
To add NOT NULL constraint to the existing column;
ALTER TABLE table_name ALTER COLUMN column_name SET NOT NULL;