title: How to subtract one table sum from other
body: <p>I have two tables: <code>Issue</code> and <code>Return</code>. Both tables have columns <code>article_id</code>, <code>person_id</code> and <code>quantity</code>. I need to subtract <code>Return</code> table from <code>Issue</code> table for one person and group the result by article name.</p>
  
  <p>First select looks like this:</p>
  
  <pre><code>SELECT Article.Name, SUM(Issue.Quantity)
  FROM Issue LEFT JOIN Article ON (Issue.ArticleID = Article.ID)
  WHERE Issue.PersonID = 2
  GROUP BY Article.Name
  </code></pre>
  
  <blockquote>
  <pre><code>ArticleName  |  Quantity
  ------------------------
  Shoes        |  5
  Coats        |  3
  Hats         |  3
  </code></pre>
  </blockquote>
  
  <p>Second like this:</p>
  
  <pre><code>SELECT Article.Name, SUM(Return.Quantity)
  FROM Return LEFT JOIN Article ON (Return.ArticleID = Article.ID)
  WHERE Return.PersonID = 2
  GROUP BY Article.Name
  </code></pre>
  
  <blockquote>
  <pre><code>ArticleName  |  Quantity
  ------------------------
  Shoes        |  3
  Coats        |  2
  Hats         |  0
  </code></pre>
  </blockquote>
  
  <p>Question is, how do I subtract second select from the first (<code>Return</code> - <code>Issue</code>) to get a table like this:</p>
  
  <blockquote>
  <pre><code>ArticleName  |  Quantity
  ------------------------
  Shoes        |  2
  Coats        |  1
  Hats         |  3
  </code></pre>
  </blockquote>
  
  <p>The Quantity in the final result should be the number of remaining articles to be returned.</p>