title: Subtract Values from Two Different Tables
body: <p>Consider table X:</p>
  
  <pre><code>A
  -
  1
  2
  3
  3
  6
  </code></pre>
  
  <p>Consider table Y:</p>
  
  <pre><code>A
  -
  0
  4
  2
  1
  9
  </code></pre>
  
  <p>How do you write a query that takes the difference between these two tables, to compute the following table (say table Z):</p>
  
  <pre><code>A
  -
  1
  -2
  1
  2
  -3
  </code></pre>