title: MySQL auto_increment on composite key no longer supported?
body: <p>I have a server running an old MySQL version (5.5.41) where the following was supported:</p>
  
  <pre><code>create table mytest33
  (
      topicnum mediumint, 
      postnum mediumint auto_increment, 
      primary key (topicnum,postnum)
  );
  </code></pre>
  
  <p>So when I insert values into this table, it would go like this:</p>
  
  <pre><code>Topicnum  Postnum
  1         1
  1         2
  2         1
  </code></pre>
  
  <p>And so forth. So the postnum value would go back to 1, if the topicnum was different.</p>
  
  <p>This is no longer supported - when I try creating the same table in MySQL version 5.6.17 I get an error: </p>
  
  <blockquote>
    <p>ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key.</p>
  </blockquote>
  
  <p>I'm using the MyISAM engine.</p>
  
  <p>How do I get around this? </p>