title: How to retrieve id's which have common values and display them together?
body: <p>I'm a beginner to SQL. I have a problem, where I have to retrieve the producer id's who have 4 or more common consumers. And I want to display the 
   Producer Id's together with the count. For Example,</p>
  
  <p>My sample database:
  <a href="http://i.stack.imgur.com/qapAY.png" rel="nofollow">Sample database</a> - my sample database</p>
  
  <p>sample output should be a and b, because they supply 4 parts in common (21,22,23,24).</p>
  
  <p>I think, I should use groupconcat and having by right?</p>
  
  <p><a href="http://i.stack.imgur.com/sEZvJ.png" rel="nofollow">Sample output</a></p>