title: MySQL "unknown"
body: <p>I just discovered <a href="http://dev.mysql.com/doc/refman/5.1/en/comparison-operators.html#operator_is" rel="noreferrer">you can write</a> something like</p>
  
  <pre><code>SELECT NULL IS UNKNOWN
  </code></pre>
  
  <p>Which returns <code>1</code>. </p>
  
  <p>Are there any other places you can use <code>UNKNOWN</code>? It doesn't appear to be a keyword (can't do <code>SELECT UNKNOWN</code>). <code>NULL IS NULL</code> is also true, so what purpose does <code>UNKNOWN</code> have?</p>