CREATE TABLE test6 (
id   NUMBER(5) NOT NULL,
f_name VARCHAR2(20),
l_name VARCHAR2(25));
 
ALTER TABLE test6 MODIFY (id NULL);
 
ALTER TABLE test6  MODIFY (id NOT NULL);
 
 
-- For example, a column in a table can be specified NOT NULL. 
-- It's not possible to insert a null in such a column. In the following 
-- create table statement, a null can be inserted into the column named c. 
 
CREATE TABLE ri_not_null (
  a NUMBER NOT NULL,
  b NUMBER     NULL,
  c NUMBER
);
 
INSERT INTO ri_not_null VALUES (   1, NULL, NULL);
INSERT INTO ri_not_null VALUES (   2,    3,    4);
INSERT INTO ri_not_null VALUES (NULL,    5,    6);