Re: One-To-One Relationships

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).

[...]

>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:
><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).

-- 
Reinier Post
TU Eindhoven