title: Use count() in CASE Expression
body: <p>I have the following data and I need help creating a case expression with a condition, When more than one PPaymentID exists per PaymentID then M is Y else false.</p>
  
  <pre><code>PaymentID   ProductID  PPaymentID
  1456789     1398        4587934
  3445738     1398        8754418 
  3445738     1399        8754419 
  </code></pre>
  
  <p>I would like to see something like </p>
  
  <pre><code>  PaymentID   ProductID  PPaymentID     M
      1456789     1398        4587934     N
      3445738     1398        8754418     Y
      3445738     1399        8754419     Y
  </code></pre>