title: Sql Returning null from second select
body: <p>I have a query like this</p>
  
  <pre><code>select * from Cars where 
  car_id!=(Select car_id from reservation where reservation_date between '2013-05-13' and '2013-05-15')
  </code></pre>
  
  <p>I want to take car_id='' if nothing between that dates but it's not working.</p>