I've fixed the problem, it's not an issue of joining tables.

1) Searching for a record in a DBGrid, using Filter = 'NR=123'
(set by customer in a TEdit)
2) Some resultfields are stored in variables
3) Filtered := false; Filter :='' when closing the Searchform.
4) When I then want to locate the record again, NR=123 is not found ...

The solution:
Set the property: AutoFetchall = true;
or TIBOQuery.InternalDataset.FetchAll after unfiltering.

This is very strange for me (coming from BDE), because the record is
located (and fetched) before. After setting Filtered to false, I have
to fetch (really all?) records again when I want to locate the record
again.

joe