title: compare datetime in oracle
body: <p>I have a commands as below:</p>
  
  <pre><code>to_char(to_date('req_date','yyyy/mm/dd'),'yyyymmdd) &gt; '20140401'; is success
  </code></pre>
  
  <p>But after when I have changed year from 2014 to 2015 , have a error format string.
  As below</p>
  
  <pre><code>to_char(to_date('req_date','yyyy/mm/dd'),'yyyymmdd) &gt; '20150401';
  </code></pre>
  
  <p>I don't know that is error.Let's fix for me.
  Thanks for all</p>