title: Remove Duplicate Rows in Oracle Table
body: <p>I want to do a SQL in Toad to extract all the values of a tables whits some conditions. I catch all the values but now I want add a new condition to the SQL but I don't know how. I want only catch the values that aren't duplicates. For example:</p>
  
  <p>I have now this:</p>
  
  <pre><code>PRINCIPAL APELLIDO NOMBRE
  a           b        c
  b           c        d
  c           d        e
  a           l        m
  </code></pre>
  
  <p>In this case I want only catch the values that <code>PRINCIPAL</code> is not duplicated, in the example I want to catch only the second and the third row because the first and the fourth are duplicated.</p>