Get the user id for triggers when deleting a user
Sounds like a job for hook_user_delete():


  Respond to user deletion.
  
  This hook is invoked from user_delete_multiple() before field_attach_delete() is called and before users are actually removed from the database.
  
  Modules should additionally implement hook_user_cancel() to process stored user data for other account cancellation methods.


e.g.

function hook_user_delete($account) {
  db_delete('mytable')-&gt;condition('uid', $account-&gt;uid)-&gt;execute();
}