title: TYPO3 conditional ext_tables.sql depends on existing table
body: <p>Usually if you want to extend a table for an extension you use something like this in <code>ext_tables.sql</code>:</p>
  
  <pre><code>CREATE TABLE tt_address (
      tx_myext_field varchar(255) DEFAULT '' NOT NULL,
  );
  </code></pre>
  
  <p>But if you havn't installed tt_address the table will be created anyway. Can I prevent this?</p>
  
  <p>If you aks now why I want to install an extension that extend another and want to prevent insert the table fields ;)</p>
  
  <p>In my case I want to create an extension that can extend the fe_user, the tt_address and other tables. But I want that the user can deside, which he want to use. fe_users, tt_address, both or other etc. It would be perfect the user can chooese this in the ext config.</p>
  
  <p>Is there any best practise for this?</p>