title: SUM of two columns from same table
body: <p>I have a table called <code>ITEM_MASTER</code> as </p>
  
  <pre><code>ITEM
  LEVEL
  </code></pre>
  
  <p>LEVEL can be 1 or 2.</p>
  
  <p>I want to find the count difference of level 2 items from level 1 items.</p>
  
  <pre><code>SUM = Level2 Items - Level1 Items
  </code></pre>