alter table tab1 modify col1 not null

Question

SQL> desc emp;
Name     Type         Nullable Default Comments 
-------- ------------ -------- ------- -------- 
EMPNO    NUMBER(4)                              
ENAME    VARCHAR2(10) Y                         
JOB      VARCHAR2(9)  Y                         
MGR      NUMBER(4)    Y                         
HIREDATE DATE         Y                         
SAL      NUMBER(7,2)  Y                         
COMM     NUMBER(7,2)  Y                         
DEPTNO   NUMBER(2)    Y                         
SQL> alter table emp modify ename not null;Table altered
SQL> desc emp
Name     Type         Nullable Default Comments 
-------- ------------ -------- ------- -------- 
EMPNO    NUMBER(4)                              
ENAME    VARCHAR2(10)                           
JOB      VARCHAR2(9)  Y                         
MGR      NUMBER(4)    Y                         
HIREDATE DATE         Y                         
SAL      NUMBER(7,2)  Y                         
COMM     NUMBER(7,2)  Y                         
DEPTNO   NUMBER(2)    Y                         
SQL> alter table emp modify ename null;Table altered

Answer 1

SQL> desc emp;
Name     Type         Nullable Default Comments 
-------- ------------ -------- ------- -------- 
EMPNO    NUMBER(4)                              
ENAME    VARCHAR2(10) Y                         
JOB      VARCHAR2(9)  Y                         
MGR      NUMBER(4)    Y                         
HIREDATE DATE         Y                         
SAL      NUMBER(7,2)  Y                         
COMM     NUMBER(7,2)  Y                         
DEPTNO   NUMBER(2)    Y