relation users to orders I want to create a list of users with their orders. I know I can create it with the Views module, but when I select user as view type, there is no relation I can use to get their orders.

Some users may not have any order. I want the list to include also those users without any order.
You can try to write a custom module that implements hook_views_data_alter() to programmatically define a relationship. See also the example code for hook_views_data().