Remove reversed tuples with same third-tier column values
<p>How can I remove reversed (m,n) &lt;-> (n,m) with the same Letter ?</p>

<pre><code>Table
id1;id2;Letter
1;2;"A"
3;4;"B"
4;3;"B" -&gt;  row to be deleted or the one above(3;4;"B"), no matter which one
5;6;"C"
6;5;"C" -&gt;  row to be deleted or the one above(5;6;"C"), no matter which one
1;2;"B"
</code></pre>

<p>Thank you everyone</p>