title: Calculate difference between two values
body: <p>I have table with 'count' column</p>
  
  <pre><code>ID    count  OperationID 
  100   111      1
  99    55       1
  94    55       1
  90    66       1 
  100   69       2
  99    33       2
  94    33       2
  90    47       2
  100   777      3
  99    55       3
  94    31       3
  90    12       3
  </code></pre>
  
  <p>I need calculate difference between two Id's counts. Need to compare ID's and values for this ID's. Ex. compare ID 100 and ID 99. EX. for ID 100 > count for ID 99 (111 > 55) show 56(difference between two values). for ID 99 > count for ID 94 (55 = 55) show 0 etc.. How can I do that?</p>