title: Explaining mysql timestamps of specific row
body: <p>I want to add at row "jailtime" timestamp.  That timestamp would be the timestamp of when "pjailed" row was updated.</p>
  
  <p>I tried to do the fallowing:</p>
  
  <p><a href="https://dba.stackexchange.com/questions/45470/get-the-time-of-last-update-of-a-column">https://dba.stackexchange.com/questions/45470/get-the-time-of-last-update-of-a-column</a></p>
  
  <p>But instead of creating table, I wanted to alter existing table, so I went for</p>
  
  <pre><code>ALTER TABLE `users` 
  CHANGE `jailtime` `jailtime` TIMESTAMP 
  ON UPDATE CURRENT_TIMESTAMP NULL 
  DEFAULT 'updated_at pJailed';
  </code></pre>
  
  <p>But it gives me error "Invalid value for pJailed"</p>