title: how to modify a constraint in sql plus?
body: <p>I have created a table as follows:</p>
  
  <pre><code>create table emp( emp_id number(5) primary key
                  , emp_name varchar(20) not null
                  , dob date );
  </code></pre>
  
  <p>After the table has been created how would I change the constraint <code>not null</code> to <code>unique</code> or any other constraint in SQL*Plus?</p>