title: Add another primary key to table in mysql
body: <p>I want to add another primary key to a table in mysql.
  I use phpmyadmin to communicate with mysql server.
  When I click the primary icon for the desired field it gives me this error:</p>
  
  <blockquote>
    <p>#1075 - Incorrect table definition; there can be only one auto column and it must be defined as a key</p>
  </blockquote>
  
  <p><strong>Edited:</strong> </p>
  
  <p>here's the query: </p>
  
  <pre><code>ALTER TABLE `files` DROP PRIMARY KEY ,ADD PRIMARY KEY ( `file_type` ) 
  </code></pre>
  
  <p>How can I do it?</p>