ORA-01442: column to be modified to NOT NULL is already NOT NULL 

ERROR

Solution : ORA-01442: column to be modified to NOT NULL is already NOT NULL

sql> alter table HR.employees modify UID not null;

ORA-01442: column to be modified to NOT NULL is already NOT NULL

SOLUTION

ALTER TABLE table_name 
  ADD CONSTRAINT constraint_name CHECK(column_name IS NOT NULL); 

OR

ALTER TABLE hr.employees 
  ADD CONSTRAINT nn_uid_hr_addrsdtls CHECK(UID IS NOT NULL);