All objects must be available through the same source connection. All database tables in a multiple-object source must have valid relationships defined by key columns or user-defined join conditions.

When you add multiple database tables as sources for a task, you can either create relationships or user-defined joins, but not both.

The

synchronization

task wizard removes a user-defined join under the following conditions:

You remove one of two remaining database tables from the list of sources for the task.
You change the source connection from database to flat file or Salesforce.