Re: circular relationships ok?
Date: Sat, 04 Mar 2006 17:58:13 -0100
Message-ID: <4409e346_at_news.fhg.de>
David Portas schrieb:
> 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.
Concept graph visualizes what the database will manage and nothing else. It represents the formal model. This can be viewed as a reference structure: if A reference B then we draw an upward arrow from A to B. You are probably talking about informal relationships in the sense of ER model. Since these relationships are not managed by the database they may have any structure including 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).
The question was how to get rid of cycles.
The problem however is that there exist different types of cycles. Informally we can (and we should) represent all of them. We can draw them like it is done in ER model. I am talking about very concrete type of cycles, namely, cycles that are known to the database and implemented via references. And for this type of relationship implemented via references we have to prohibit cycles in order to get significant advantages in automating data management. All other types of relationships that are not managed by the database may have any desirable structure and constraints. Thus we may and we need to model cycles and even more complex relationships but we have to do it without cycles in reference structure. Why? Because the database will not be able to manage such data (only trivial operations are possible). With no cycles the database is able to manage the data as a global interrelated structure.
-- http://conceptoriented.comReceived on Sat Mar 04 2006 - 19:58:13 CET