title: Set default value as column value
body: <p>i want to set default value as column for existing table, 
  example : <code>create table t(a Number(22), b NUMBER(22)</code>;</p>
  
  <p>i jsut want to set default value for <code>column B as column A</code>.</p>
  
  <p>I know we cannot do like this, could any one suggest an alternate way to do so..</p>