Re: One-To-One Relationships

From: JOG <>
Date: Sat, 1 Dec 2007 06:03:13 -0800 (PST)
Message-ID: <>

On Dec 1, 11:08 am, "David Cressey" <> wrote:
> "paul c" <> wrote in message
> news:et24j.80277$cD.13174_at_pd7urf2no...
> > JOG wrote:
> > ...
> > > Why don't you first define the term entity, as opposed to just
> > > assuming their existence as axiomatic. I look forward to there being

> > > absolutely no room for debate or disagreement! Look, you get a lot of
> > > respect on this forum, particularly off myself, but this does not
> > > excuse arrogance, so now we are all ears.
> > > ...
> > PMFJI but this is just too much fun not to have a go: Something that
> > exists?
> Anything that has been reified, and whose instances display haeccity.
> Sorry, I couldn't resist.

And not a mention of quiddity. tut!

Although I have no doubt over-reacted to Jan's dig at the discussion (ah, the cold light of day), let me give an example as to why I find the breakdown into entities and relationships deleterious. Say I have two entity types staff_members and subjects, and a relationship teaches:

staff_member -- teaches --> subject

This is all good and fine until a requirement changes that we need to record the day the lecture is given on. To denote the is new information, well I now longer haver a binary relationship, but a ternary one, and that requires a rewrite of the E/R representation (given that it is a graph). I need a new lecture associative entity with 'day' as an attribute.

staff_member -- holds --> lecture <--- involves -- subject

There is no need for such brittleness in the face of what is a relatively trivial schema change. If one views relationships and entities on the same keel to start with its just a simple case of adding an extra attribute. The problem is that E/R views binary relationships as somehow a different species from n-ary ones, and Kent critiqued this very issue in his excellent book 'data and reality' 30 years ago.

We describe entities via propositions. We describe relationships via propositions. Surely there is a common theme there. Received on Sat Dec 01 2007 - 15:03:13 CET

Original text of this message