Re: "Structured" Entity-Relationship Model?

Wolfgang Keller wrote:

> Hello,
>
> coming from the "practitioner" side, I wonder what relational theorists
> think about "structured entity relationship" modelling:
>
> http://en.wikipedia.org/wiki/Structured-Entity-Relationship-Model
>
> For me the essential aspect seems to be the rule (which doesn't even
> seem to be mentioned in the english-language Wikipedia article) that the
> dependency graph of all entities must not contain directed circles,
> i.e. there must be no "hen and egg"-type dependencies between entities.

I can't think why that would be important.

> Which appears to be a perfectly reasonable rule to me, since such
> dependencies create horrible headaches for working with such databases.

Really? Like what? I suppose if the DBMS insists of testing the referential integrity contstraints before you assert that you have completed a transaction that has left the database consistent (i.e. you COMMIT) that would be a problem, but that's just a broken DBMS. Indeed most SQL DBMSs seem to want to test constraints after every blessed statement, which is silly.

I don't really applaud products/methodologies that try to elevate the workarounds for implementation mis-features to "best practice", as it appears SERM does.

> Just imagine loading or deleting bulk data with circular dependencies.

I have no problem imagining that at all. SQL applies a series of tiny, local, sequential changes to the database. It is absurd to expect every such incremental change must leave the database consistent. It is absurd that one could not do "bulk loading" or "bulk deleting" or any other series of things, in any random order, deferring the question of consistency until you, the programmer, want to make that claim.

> Now what do theorists think:
>
> Is it perfectly evident that this requirement must be enforced, since a
> model with cyclic dependencies is plain "spaghetti", maybe even
> violating some normal form?

No.

> Is it plain nonsense?

I'm not sure what "it" is here.

> Or does it depend on
> the specific case, since there may be situations where it can't be
> achieved, and a model that violates this rule can be perfectly valid?
>
> I've noted that in practice, graphical modelling tools seem to be prone
> to making users produce models where this rule is violated.

Most modelling tools are rubbish, but in this particular respect I don't see a problem. Quite the contrary.

> There
> don't seem to be modeling tools that would allow checking for such
> dependencies resp. preventing them.

Good.

-- 
Roy