title: Calculate previous year based on this year condition
body: <p>Let's say i have a table called Orders with number of units sold by resellers in Country by date</p>
  
  <pre>
  Country ResellerID Date   Units
  3       1          May-17 7
  3       1          May-16 4
  3       2          May-16 3
  2       3          May-16 5
  2       3          May-17 8
  </pre>
  
  <p>I have to compare total units sold in specific month basing on a slicer selection to the last year total units in the corresponding month. This is relatively easy and I do this using the formula below</p>
  
  <p>UnitsLastYear = CALCULATE(SUM(Orders[Units]),SAMEPERIODLASTYEAR(Orders[Date]))</p>
  
  <p>So for May-17 chosen on the slicer I have:<br/>
  This year = 15<br/>
  Last Year = 12<br/></p>
  
  <p>However there is another condition for calculating last year's value and this is that I should only take Resellers that sold anything in current year's selected month so in this example ResellerID = 2 should be excluded from last year calculation and Last Year should have value of 9</p>
  
  <p>Does anyone can help how to transform this calculation so that it would rflect this condition?</p>