title: Oracle NVL with empty string
body: <p>I have this table where NULL is the <code>NULL</code> value, not the string NULL:</p>
  
  <pre><code>MYCOL
  --------
  NULL
  example
  </code></pre>
  
  <p>Why does this query <em>not</em> return the <code>NULL</code> row?</p>
  
  <pre><code>select * from example_so where nvl(mycol, '') = '';
  </code></pre>