title: Triple Table Join with Subtraction
body: <p>I have 3 tables (production, sales, wastage)<br>
  All 3 tables have the columns: Product and Quantity<br>
  I need a query statement that will join all 3 tables in the following manner:<br><br>production - sales - wastage<br><br> Grouped by Product</p>
  
  <p>Thanks in advance for the tips and tricks :) </p>