title: MySQL 5.5 Alter Table TimeStamp default to Current Value, but it's being set to 0 instead
body: <p>I am trying to add a <code>last_updated</code> column to a table w/ preexisting data in it. After I execute the <code>ALTER TABLE</code> statement, I expect for the <code>last_updated</code> column for all existing data to be set to the <code>CURRENT_TIMESTAMP</code>.  </p>
  
  <pre><code>ALTER TABLE TableName
  ADD COLUMN last_updated TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP 
  ON UPDATE CURRENT_TIMESTAMP
  </code></pre>
  
  <p>However, the value is set to <code>0000-00-00 00:00:00</code> for all preexisting rows. Any subsequent inserts are created with <code>CURRENT_TIMESTAMP</code> as expected. </p>
  
  <p>I am using MySQL 5.5. Are my expectations incorrect here? Or am I doing something wrong?</p>