title: stop duplicates when one part of the same record exists
body: <p>i have a table called registry  and i have a couple of columns, for this example i am only going to state four(4) columns:-</p>
  <ul>
  <li>r_id</li>
  <li>file_number</li>
  <li>file_name</li>
  <li>volume</li>
  </ul>
  <p>now these columns support duplicate filenumbers and filename but for each duplicate there needs to be a different volume. for eg.</p>
  <pre><code>r_id              filenumber            filename             volume
  1                  123                   test                  1
  2                  123                   test                  2
  3                  234                   oracle                1
  4                  234                   oracle                2
  5                  123                   test                  1
  </code></pre>
  <p>now the issue is <strong>r_id 5</strong> which  has a duplicate volume as <strong>r_id 1</strong>.</p>
  <p>so my question is how can i stop a duplicate volume when a record with the same file_number , file_name and volume exist?.</p>
  <p>Nb.I am using the regular insert statement to create the records</p>