oracle - how to modify a constraint in sql plus? -

i have created table follows:

create table emp( emp_id number(5) primary key                 , emp_name varchar(20) not null                 , dob date ); 

after table has been created how change constraint not null unique or other constraint in sql*plus?

you don't change constraint 1 type another. can add unique constraint table

alter table emp    add ( costraint uk_emp_name unique( emp_name ) ); 

that independent of whether emp_name allowed have null values.