>>> Why did OO database in the late 90´s did not break through?
>
>>Because we didn't have XML or XSD yet.
>
>I don´t think it is because of the storage of data or the carrier, but more about SQL which is the standard DML for databases. There is no standardized DML for OO stored data.

Right. I'm not suggesting XML or XSD is the answer because it lets us store or transfer the data in unique ways, I'm saying we can use XSD as the DML! The equivealent of relational SQL statements could be written in XML resulting in objects: you need these fields from these aliases related in these ways filtered by these conditions and sorted in these orders. You stick that information into a higherarchal XML document and walla, thats the whole business object.

>How do you implement referential integrity in a OO design etc?

Part of the schema. The schema is used to generate source code for classes which does all the RI.