title: MySQL insert where not exists / if not exists
body: <p>I have tried the following query:</p>
  <pre><code>INSERT INTO `surfed_site` (user, site)
  VALUES ('123', '456')
  WHERE NOT EXISTS (SELECT site FROM `surfed_site` WHERE site=456)
  </code></pre>
  <p>But I keep getting a MySQL error:</p>
  <p><strong>1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'WHERE NOT EXISTS (SELECT site FROM <code>surfed_site</code> WHERE site=456)' at line 3</strong></p>
  <p>I have no clue what I'm doing wrong, would anybody be able to guide me?</p>