title: SQL Select unique combinations of rows for other column value
body: <p>I’m trying to do an analysis of the different combinations of taxes per invoice to identify how many scenarios exist. 
  In the tax table, column 1 is invoiceNo, column 2 is taxType. These form the composite key. There can be 1 or more taxType per invoiceNo. Example of data:</p>
  
  <p><a href="https://i.imgur.com/bcQc7vY_d.jpg?maxwidth=640&amp;shape=thumb&amp;fidelity=medium" rel="nofollow noreferrer">https://i.imgur.com/bcQc7vY_d.jpg?maxwidth=640&amp;shape=thumb&amp;fidelity=medium</a> (Sorry but i’m new so can’t add picture).</p>
  
  <p>I want to be able to report on unique taxType for any invoiceNo. Ie, 1 A is unique comb 1, 2 AB is unique comb 2, 3 A is disregarded as already returned for 1, and 4 BC is unique comb 3. </p>
  
  <p>Not sure if this makes sense! Finding it hard to articulate what I’m after! </p>
  
  <p>Expected output would be:</p>
  
  <pre><code>A
  AB
  BC
  </code></pre>