title: mysql - drop unique constraint
body: <p>In my table I have a foreign key which is also a unique key.
  How can I remove the unique Key without removing the foreign key?</p>
  
  <p>When I do : </p>
  
  <pre><code>ALTER TABLE affaire DROP KEY contact_client_id;
  </code></pre>
  
  <p>I am getting a : <code>ERROR 1553 (HY000): Cannot drop index 'contact_client_id': needed in a foreign key constraint</code></p>