Without much detail on the app architecture, I cannot offer much help.
It does sound like you have the data available to relate the tables - but that would require a key field change, which may not be a viable option.
table A - Date - Van
table B - Date - Van
If you could relate Table A records to child Table B records via the date and Van id - you could look up the driver easily.
If you cannot add to the API that writes the records to relate to Table A, perhaps you can create a Table to Table import that will merge on a custom Date/Van key. This will write the RID of table A to any records that match the key in table B in the reference field of the relationship. You can trigger this import manually, or via an automation. Maybe a daily automation to run the import would be sufficient, rather than a triggered event.
Let me know if this helps.
