Find empty portal records
A simple thing (I assume) but I have searched and can't find it.
I have records (customers) with a related record portal (bookings) - I want to find those records who have no portal records. The 'obvious' way - to type '=' in the unique ID doesn't work because, of course, all related records have an ID.
So how do I find those records with an empty portal ie customer who have made no bookings?