Re: One-To-One Relationships
Date: Thu, 29 Nov 2007 14:16:48 -0800 (PST)
On Nov 28, 8:43 pm, rp..._at_pcwin518.campus.tue.nl (rpost) wrote:
> paul c wrote:
> >Regarding ER, here are some quotes from Codd's book (available for free
> >at acm.org). The sarcasm of the second one made me laugh.
> The criticisms you quote may be amusing, and they have merit, but
> they ultimately miss the point. The distinction between entities and
> relationships: entities have identity (they can be referred to; attributes
> can have entity-valued domains), while relationships do not (they are
> completely identified by their, possibly entity-valued, attributes).
I realise that others have attempted to point out your mistakes, but I just wanted to echo their sentiment. There is absolutely no difference between an entity and a relationship. E/R modelling has /itself/ conceded this, translating relationships into "associative entities".
The classic example of this is a "marriage" entity.
> >3. Even if this distinction had been precisely defined, it would have added
> >complexity without adding power. Whatever is conceived as entities, and
> >whatever is conceived as relationships, are perceived and operated
> >upon in the relational model in just one common way: as relations.
> This is the exact problem Chen identified. In the relational model
> it is impossible to have entity-valued attributes, which, in practice,
> we have a huge amount of. 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.
> 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.)
> > From chapter 30:
> >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).
> Reinier Post
> TU Eindhoven
Received on Thu Nov 29 2007 - 23:16:48 CET