title: mysql count unique row values
body: <p><code>TABLE quotation</code></p>
  
  <pre><code>id  clientid
  1   25
  2   25
  3   25
  4   25
  5   26
  </code></pre>
  
  <p>How can I query how many different clients exist in <code>TABLE quotation</code>? I don't want duplicate entries to be counted more than once.</p>
  
  <p>I need the answer to be <code>2</code>, in 1 row, because the only non-duplicated entries are (<code>25</code>, <code>26</code>).</p>