title: MySQL copy values of one column to another column
body: <p>I have two tables:</p>
  
  <ul>
  <li><code>t1(mainid, parentid)</code> </li>
  <li><code>t2(mainid, parentid)</code></li>
  </ul>
  
  <p>How would I copy the values of <code>t1.mainid</code> to <code>t2.parentid</code>?</p>
  
  <p>I tried using: </p>
  
  <pre><code>SELECT mainid INTO t2 FROM t1
  </code></pre>
  
  <p>but it does not work.</p>