title: Select all items in a table that do not appear in a foreign key of another table
body: <p>Take for example an application which has users, each of which can be in exactly one group.  If we want to SELECT the list of groups which have no members, what would be the correct SQL?  I keep feeling like I'm just about to grasp the query, and then it disappears again.</p>
  
  <p>Bonus points - given the alternative senario, where it's a many to many pairing, what is the SQL to identify unused groups?</p>
  
  <p>(if you want concrete field names:)
  One-To-Many:</p>
  
  <pre><code>Table 'users': | user_id | group_id |
  Table 'groups': | group_id |
  </code></pre>
  
  <p>Many-To-Many:</p>
  
  <pre><code>Table 'users': | user_id |
  Table 'groups': | group_id |
  Table 'user-group': | user_id | group_id |
  </code></pre>