title: Get all usernames stored in TinyDB database
body: <p>I'm new to TinyDB, and new to Python.</p>
  
  <p>I have various usernames stored in a TinyDB database, they also have other information stored (age, email addresses etc), however, I wish to return all the usernames only. </p>
  
  <pre><code>{"_default": {"1": {"Username": "John", "Age": "30"}, "2": {"Username": 
  "Andrew", "Age":"40", "Email": "example@example.com"}}}
  </code></pre>
  
  <p>My GUI would have the button "Show all usernames".</p>
  
  <p>I can return information about <em>specific</em> users, and I can get <em>all</em> the information stored in the database (db.all()), however I cannot just seem to get all the usernames from the entire database.</p>
  
  <p>Is there a way to do this?</p>
  
  <p>Or am I looking at this problem the wrong way.</p>
  
  <p>Many thanks! </p>