Re: "Structured" Entity-Relationship Model?

Op zondag 21 april 2013 17:45:11 UTC+2 schreef Roy Hann het volgende:
> Wolfgang Keller wrote:
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> > Hello,
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> >
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> > coming from the "practitioner" side, I wonder what relational theorists
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> > think about "structured entity relationship" modelling:
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> >
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> > http://en.wikipedia.org/wiki/Structured-Entity-Relationship-Model
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> >
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> > For me the essential aspect seems to be the rule (which doesn't even
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> > seem to be mentioned in the english-language Wikipedia article) that the
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> > dependency graph of all entities must not contain directed circles,
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> > i.e. there must be no "hen and egg"-type dependencies between entities.
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> I can't think why that would be important.
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> > Which appears to be a perfectly reasonable rule to me, since such
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> > dependencies create horrible headaches for working with such databases.
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>
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> Really? Like what? I suppose if the DBMS insists of testing the
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> referential integrity contstraints before you assert that you have
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> completed a transaction that has left the database consistent (i.e. you
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> COMMIT) that would be a problem, but that's just a broken DBMS. Indeed
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> most SQL DBMSs seem to want to test constraints after every blessed
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> statement, which is silly.
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> I don't really applaud products/methodologies that try to elevate the
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> workarounds for implementation mis-features to "best practice", as it
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> appears SERM does.
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> > Just imagine loading or deleting bulk data with circular dependencies.
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> I have no problem imagining that at all. SQL applies a series of
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> tiny, local, sequential changes to the database. It is absurd to
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> expect every such incremental change must leave the database
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> consistent. It is absurd that one could not do "bulk loading" or "bulk
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> deleting" or any other series of things, in any random order, deferring
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> the question of consistency until you, the programmer, want to make that
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> claim.
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> > Now what do theorists think:
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> >
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> > Is it perfectly evident that this requirement must be enforced, since a
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> > model with cyclic dependencies is plain "spaghetti", maybe even
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> > violating some normal form?
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> No.
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> > Is it plain nonsense?
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> I'm not sure what "it" is here.
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> > Or does it depend on
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> > the specific case, since there may be situations where it can't be
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> > achieved, and a model that violates this rule can be perfectly valid?
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> >
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> > I've noted that in practice, graphical modelling tools seem to be prone
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> > to making users produce models where this rule is violated.
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> Most modelling tools are rubbish, but in this particular respect I don't
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> see a problem. Quite the contrary.
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> > There
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> > don't seem to be modeling tools that would allow checking for such
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> > dependencies resp. preventing them.
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> Good.
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>
>
> --
>
> Roy

Is the statement-level constraint checking still "silly" if multiple assignment as per TTM is available ? Received on Tue Apr 30 2013 - 17:06:50 CEST