Re: One-To-One Relationships

From: paul c <>
Date: Wed, 28 Nov 2007 22:08:13 GMT
Message-ID: <hFl3j.62890$cD.4080_at_pd7urf2no>

rpost wrote:
> This is the exact problem Chen identified. ...

Maybe he did identify a problem that some people might have, but I don't see how one can say they've defined much of a model unless some operators have been defined that aren't particular to one application, ie., not pet verbs that pop out of somebody's head at a requirements meeting.

> In the relational model
> it is impossible to have entity-valued attributes, which, in practice,
> we have a huge amount of.

If by entity-valued attributes you mean so-called EAV, I think the archives contain plenty of other people's comments about that, maybe even too many!

> When I say a particular person is the head
> of a particular department, I do not mean to assert a relationship
> between particular combinations of attribute values for departments
> and persons; I may not even know what these attributes are.

In that case, the Information Principle is being thrown out the window, OTOH it's a good excuse to avoid writing the app!

> I really want to establish a link between the department itself
> and the person herself, whatever their identifying attributes.
> The most important thing Chen says is: let's express this type
> of relationship directly, instead of relying on a particular, and
> necessarily imperfect, way to simulate them in the relational model.
> (Surrogate keys are good enough, but not perfect.)

Forget surrogates, attributes are good enough, just as perfect as human comprehension and probably equally as perfect as human expression, outside of poetry of course.

>> From chapter 30:
>> <quote>
>> Of the five approaches discussed in this chapter, this one is clearly
>> the winner in terms of its lack of precise definitions, lack of a clear
>> level of abstraction, and lack of a mental discipline. The popularity
>> of ER may lie in its multitude of interpretations, as well as its use
>> of familiar but obsolete modes of thought.
>> <end quote>

> The complaint that makes sense here is the lack of precise definitions.
> When working with ER modelling (based on the Silberschatz et al. textbook)
> I haven't found that to be a serious problem. I never saw ambiguities
> in interpreting notions, except in features that I consider too unsanitary
> to use (e.g. cardinalities on non-binary relationships).

Heh, I don't think it's a problem either, that is not until one decides that's enough of the ER modeling, donuts and pleasantries, throwing pet terminology and other BS around and tries to implement it. With sufficient refreshment on hand, I think it's a fair device for locking up some people in a room who can't describe exactly what they do and don't want to be there in the first place. (OTOH, I read somewhere about the mental patterns of long-time chess masters and how they lose the ability to describe exactly how they make some of their playing decisions. That would be some ER "meeting" or whatever the fashionable term for it is these days!) Received on Wed Nov 28 2007 - 23:08:13 CET

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