title: Self-join on a non-key column
body: <p>Are there are analytical consequences on <code>JOIN</code>ing a table to itself <code>ON</code> a column that does not hold unique values?</p>
  
  <p>Most of the self join queries I have seen are done on a <code>PK</code> column. I have run into problems getting my numbers right so I am wondering if what I am doing makes sense.</p>
  
  <p>E.g.</p>
  
  <pre><code>SELECT * 
  FROM Employee a
  JOIN Employee b
  ON a.Birthdate = b.Birthdate
  </code></pre>