title: Mysql auto-initialization of timestamp column when adding it to an existent table
body: <p>I want to add a column to a table to keep track of when each record was last updated. I already have some data in the table.</p>
  
  <p>The SQL query I am running is this:</p>
  
  <pre><code>ALTER TABLE `my_table` ADD COLUMN `last_modified` timestamp NOT NULL 
            DEFAULT CURRENT_TIMESTAMP 
            ON UPDATE CURRENT_TIMESTAMP`;
  </code></pre>
  
  <p>When adding columns of other types existent records get the default value I specify for that column. However in this case all existent records are getting assigned the value of: <code>0000-00-00 00:00:00</code>
  Why is this happening if I explicitly said I wanted the default to be the current timestamp?</p>