title: How to change datatype of column with data
body: <p>Let's say I have a table <code>call_log</code> that has a column named <code>duration</code>. Let's further pretend that when I put the table together, I made the mistake of making <code>duration</code> a varchar column instead of a number.  Now I can't properly sort on that column. I want to rename the column so I issue...</p>
  
  <pre><code>ALTER TABLE call_log MODIFY (duration NUMBER);
  </code></pre>
  
  <p>But I get...</p>
  
  <pre><code>ORA-01439: column to be modified must be empty to change datatype.
  </code></pre>
  
  <p>My table has been in use and has data! And I don't want to lose the data. How do I modify the column's datatype without losing data?</p>