title: Subtract two result sets SQL
body: <pre><code>Result1     Result2
  a           a
  b           b    
  a
  </code></pre>
  
  <p>How to obtain <code>a</code> by doing a subtraction between these two result sets. <code>NOT EXISTS</code> and <code>NOT IN</code> return nothing for these two result sets. But i want <code>a</code> to be returned. Pleas help!   </p>