title: Powerbi DAX comparing two values in the same table with the same id
body: <p>I need some help with comparing two values in the same table and finally subtracting them. The data is imported from sharepoint using two different excel files both containing data from a different month.</p>
  <p>I have created a mock table of my problem.</p>
  <p><a href="https://i.stack.imgur.com/SsCs0.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SsCs0.png" alt="" /></a></p>
  <p>This is how it looks after the changes</p>
  <p><a href="https://i.stack.imgur.com/b04eV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/b04eV.png" alt="enter image description here" /></a></p>
  <p>What I need is a new measure that calculates the difference between item_new and item_old where the id is the same. I'm a beginner on dax formulas and currently don't know how to solve this problem.</p>