You can create an advanced relationship for database sources when the source object in the mapping is a parameter and configured for multiple sources. You cannot create an advanced relationship between source objects that have been joined using a custom relationship.
When you create an advanced relationship, the wizard converts any relationships that you defined to an SQL statement that you can edit.
To create an advanced relationship, you add the primary source object in the Objects and Relationships table. Then you select fields and write the SQL statement that you want to use. Use an SQL statement that is valid for the source database. You can also add additional objects from the source.