Re: circular relationships ok?

Alexandr Savinov wrote:

> Now for each order there as an invoice and vice verse (we need of course
> a constraint to enforce one-to-one relationship).

So you DO allow constraints that are cyclical. This constraint (called "N-cardinality", where N=1) applies "in both directions". In a conventional ER diagram this would be shown as a cycle. Your diagrams on the other hand don't seem to model constraints - they show something else that isn't clear to me. That something else doesn't have any cycles.

The OPs original question was specifically about cycles in referential integrity constraints, which you do say you need in your model (you just don't draw pictures of them).

--
David Portas