title: using alter to add column with specific definition
body: <p>how do i add a column to an existing table with a given definition. For example i have a table called employee with column called hire_date. Now using alter and <strong>only</strong> alter, i want to add a column called tenure which contains the difference between (hire_date and 14-JULY-2012). this is what ive tried</p>
  
  <pre><code>alter table employee add(tenure numeric(10) where tenure=(select to_date(
  '14-JULY-2012')-HIRE_DATE from employee));
  </code></pre>