title: Is this achievable using left joins in SQL query?
body: <p>I have two tables:</p>
  
  <p><strong>t_voucher</strong></p>
  
  <ul>
  <li>id</li>
  <li>title</li>
  <li>description   </li>
  </ul>
  
  <p><strong>t_used_voucher</strong></p>
  
  <ul>
  <li>id</li>
  <li>customerid</li>
  <li>date_used</li>
  </ul>
  
  <p>I want to get results for all vouchers that have NOT been used by <code>customerid</code></p>
  
  <p>Is this even possible in SQL or an alternative approach ?</p>
  
  <p>The closest, I can get is something like this, but I don't think I can filter by <code>customerid</code></p>
  
  <pre><code>SELECT
      t_voucher.id, t_voucher.title,
      t_voucher.description,
      t_used_voucher.customerid
  FROM
      t_voucher
  LEFT JOIN 
      t_used_voucher ON t_voucher.id = t_used_voucher.id
  WHERE
      t_used_voucher.id IS NULL
  </code></pre>