title: Column to be modified is not an identity column
body: <p>I have created a table with column <code>S_ROLL NUMBER(3) NOT NULL</code> Now I want to make this colum to as identity column.
  I used this command</p>
  
  <pre><code>alter table students
  modify
  (
  S_ROLL NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY
  );
  </code></pre>
  
  <p>Then I'm getting this error.</p>
  
  <pre><code>S_ROLL NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY
  *
  ERROR at line 4:
  ORA-30673: column to be modified is not an identity column
  </code></pre>