title: Virtual column definition referring to another virtual column
body: <p>Is it possible to have a virtual column whose column expression definition refers to another virtual column, that has previously been defined on the table? <a href="http://docs.oracle.com/cd/E11882_01/server.112/e41084/expressions005.htm#SQLRF20043" rel="nofollow">The documentation for column expressions</a> says:</p>
  
  <blockquote>
    <p>the defining <em>column_expression</em> must refer only to columns of the subject table that have already been defined, in the current statement or in a prior statement</p>
  </blockquote>
  
  <p>This is a simple example of what I'm trying to do, with the third virtual column referring to other <em>virtual</em> columns that have already been defined:</p>
  
  <pre><code>create table numberManipulation
  (
    value1 VARCHAR2 (10),
    Value2 VARCHAR2 (10),
    Addition AS  (DECODE (value1, 'late', 1, 0) + DECODE (value2, 'late', 1, 0)),
    subtraction AS(DECODE (value1, 'present', 1, 0) + DECODE (value2, 'present', 1, 0)),
    Compile AS (ADDITION + SUBTRACTION)
  );
  </code></pre>
  
  <p>But this gets error:</p>
  
  <pre><code>ORA-54012: virtual column is referenced in a column expression
  </code></pre>
  
  <p>Is what I'm trying to do possible?</p>