title: Why MYSQL IN keyword not considering NULL values
body: <p>I am using the following query:</p>
  
  <pre><code>select count(*) from Table1 where CurrentDateTime&gt;'2012-05-28 15:34:02.403504' and Error not in ('Timeout','Connection Error');
  </code></pre>
  
  <p>Surprisingly, this statement doesnot include the rows having Error value as NULL.My intention is to filter only rows with Error value as 'Timeout' (or) 'Connection Error'. I need to give an additional condition( OR Error is NULL) to retrieve the correct result.</p>
  
  <p>Why is MYSQL filtering out results with NULL values?
  I thought that IN keyword would return a boolean result (1/0) and now i understand that some MYSQL keywords doesnt return boolean values,it might return NULL too....but Why is it treating NULL as special?</p>