title: SSRS Subtracting union columns
body: <p>I've hit a bit of a wall while building a report...</p>
  
  <p>This is my underlying query</p>
  
  <pre><code>SELECT        'unionCol1Name' AS SRC, col1,col2,col3
  FROM            tbl1
  UNION ALL
  SELECT        'unionCol2Name', col1,col2,col3
  FROM            tbl2
  </code></pre>
  
  <p>I have a matrix report that displays both of these union columns but only when viewed. My problem is that I need to subtract one column from the other but I'm having difficulty defining the columns as they're in a union.</p>
  
  <p>I have a feeling there's a way to distinguish between the two union columns that are generated once the report has run, through the textbox expression field in the report design.</p>
  
  <p>The report looks like this once previewed:</p>
  
  <pre><code>union1 results    Union2 results
  78                   54
  12                   24
  51                   12
  63                   43
  </code></pre>
  
  <p>Any help is much appreciated.</p>