title: how to select max of mixed string/int column?
body: <p>Lets say that I have a table which contains a column for invoice number, the data type is VARCHAR with mixed string/int values like:</p>
  
  <pre><code>invoice_number
  **************
      HKL1
      HKL2
      HKL3
      .....
      HKL12
      HKL13
      HKL14
      HKL15
  </code></pre>
  
  <p>I tried to select max of it, but it returns with "HKL9", not the highest value "HKL15".</p>
  
  <pre><code>SELECT MAX( invoice_number )
  FROM `invoice_header`
  </code></pre>