title: SQL query for getting active employees in specific period
body: <p>Having the following table:</p>
  
  <pre><code>    ID     EmployeeID      Status       EffectiveDate
    ------------------------------------------------------
       1       110545        Active        01AUG2011
       2       110700        Active        05JAN2012
       3       110060        Active        05JAN2012
       4       110222        Active        30JUN2012
       5       110545        Resigned      01JUL2012
       6       110545        Active        12FEB2013
  </code></pre>
  
  <p>How do I get the number of active (or partially active) in a specific period? 
  For example, if I want to know all active (or partially active) employees from <code>01JAN2011</code> to <code>01AUG2012</code> I should get 4 (according to the table above). If I want to know all active employees from <code>01AUG2012</code> to <code>01JAN2013</code> it should be 3 only (because employee 110454 is resigned). </p>
  
  <p>How will I do that?</p>