title: How do constant values effect the ON clause of Joins?
body: <p>I've recently discovered that the ON clause of a LEFT JOIN may contain values such as (1 = 1).</p>
  
  <p>This is upsetting to me, as it breaks my perception of how joins function.</p>
  
  <p>I've encountered a more elaborate version of the following situation:</p>
  
  <pre><code>SELECT DISTINCT Person.ID, ...
  FROM Person LEFT JOIN Manager 
  ON (Manager.ID = Person.ID OR Manager.ID = -1))
  WHERE (...)
  </code></pre>
  
  <p>It's perfectly legal.  What does "Manager.ID = -1" accomplish, if anything? How can this effect the Join?</p>