If you perform a find after entering an asterisk into a never empty field in the related table occurrence, it will find all records that have a related record. If you make that an Omit find request, you find all records that do not have any related records.
I have to admit I'm not entirely sure how to perform the find you are specifying. I've attached a screenshot of the relevant area of my database, which might be more clear than me trying to explain it.
I am not sure which pair of table occurrences represent the pair in question. (And being ill is definitely affecting my ability to think clearly.)
Here's an example using my own table occurrences.
Say you have this relationship:
Parent::__pkParentID = Child::_fkParentID
You want all Parent records that are not linked to at least one Child record. (In your self join, these will be the "root" observations.)
Enter find mode on a Parent Layout
Put an asterisk (*) in the Child::_fkParentID field
Click the Omit button
Perform the find
Any field in Child that is never empty will work for the asterisk. Since the foreign key field will never be empty for related records, this makes it a good choice for me to specify in this example.
The above find can be manual or scripted. See the following link for scripted find examples: Scripted Find Examples
Oh man, that was so obvious once you explained it that way. Thanks again.