Hi Hans,
I am an ex iPhone user.
There was a fantastic relational database, called iSort.
But I don't know wether it is still being updated.
This other solution is Handbase for iPhone or Handbase for iPad, with their eqivalent desktop apps.
But this is in my opinion less intuitive than Memento DB.
I am new at discovering it but already love Memento.
Hope that helps,
D'Jack