I don’t think there is a direct function for this. But you can loop through the second table inside main table loop and mark the records in main table which are not matching with the record in second table
You could create a flowfield in the main table of type Exist looking up if a record in the subtable exists. Then simple setrange the flow field = false. Then the result set will be the records without a sub-record.
In the new NAV versions it will even create a single SQL join statement so the performance might be ok.