Re: Has E/R had a negative impact on db?
- From: "Jan Hidders" <hidders@xxxxxxxxx>
- Date: 24 Apr 2006 15:25:43 -0700
JOG wrote:
Jan Hidders wrote:
JOG wrote:
Just a thought.
Likewise. :-)
I don't like entities. In fact I despise entities, as the enemy of good
information philosophy. You see I just don't accept their existence.
Actually, I don't see what you could possibly mean with that, and to
the extent that I can guess a meaning I find that a rather baffling
statement.
Agreed, in a scientific paper, I would be embarressed by such nonsense,
so take it with a pinch of salt - the prose is purple to engender
responses, however one wishes to interpret such a rant.
Ok. Fair enough.
The point I am
really making is that I see no satisfactory distinction between
entities and relationships, that I find this jarring, and that perhaps
making this split may not always be productive. Nonetheless, from this
thread I have discovered ORM and have been pretty impressed by it.
Indeed. One of the few ER dialects that took into account that
sometimes you would like to consider the same thing as both an entity
and a relationship.
The usual definition of 'entity' in the context of data
modelling for database is something like "things that can be identified
and which we would like to store information about in the database". If
that's how I interpret your statement then you seem to be saying that
you don't believe there are such things, and since we cannot store
information about things we cannot identify, this leads inexorably to
the conclusion that you don't believe that we can store information
about anything interesting in a database.
Right, so your working definition is that entities are "things that can
be identified and which we would like to store information about in the
database". Relationships however are also things, they can be identifed
and we would like to store them - so the definition appears to be on a
sticky wicket.
Yes, relationships are a special kind of entity. Why do you think this
is problematic? I would argue that this is not a deep problem in ER
modelling and can be fixed. ORM, for example, does that to some extent.
There is no magical wrapper surrounding some construct that turns it
into a nicely formed 'thing' or 'object'.
What makes you think a wrapper is needed?
The use of the term 'entity' has seemed to me to be as a collection of
attributes or properties, wrapped up together. Objects equally are
wrappers in that they encapsulate whatever is inside. I may have missed
your point - I was just inferring that these groupings of attributes
are made in pragmatic fashion and that there is no right or wrong to
them.
Encapsulation is a nonsensical concept in the context of data
modelling. It looks a bit as if the problem is more that you are
retro-fitting terminology from the world of object-oriented programming
into the world of datamodelling that has no place there. ;-) More to
the point: it's certainly not an inherent part of modelling with ER
dialects.
What makes you think a relationship is necessarily binary? And why do
you think it is a problem that the distinction between relationsips and
entities is not clear?
Well, I offered the suggestion that this may have encouraged a
navigational viewpoint of data, between nodes connected by links.
Binary relationships encourage just as much a navigational viewpoint as
foreign keys do. And even if they did and the query language would
encourage you to specify queries in a navigational way then this
becomes only problematic if the query engine is going to execute
navigationally specified queries in a navigational way.
Nothing. In fact all they are, are special cases of n-ary
relationships, or 'associative entities'.
No, that is clearly false. Not every entity is necessarily identified
by its associated attributes, identification could be more complex
and/or indirect than that.
Well, clearly not clearly. Would liebniz equality not state that the
only thing that identifies an item is very much its atrributes?
You mean Leibniz' substitution principle?:
Two things X and Y are equal iff in all true propositions containing X
we can substitute Y for X and again obtain a true propsition and vice
versa.
That would only imply what you think it implies if only attributes
where stored, and not relationships.
Given
an E/R relationship can always be restated as an associative entity I'd
appreciate if you could expand on this.
Consider weak entities, say the weak entity "Dog" with attribute "name"
that depends via the "is-owned-by" relationship upon the entity "Owner"
with attributes "name" and "address". To identify a dog you not only
need its name (which is probably not globally unique) but also the name
and address of its owner.
Serious study of object / entity identity would
have led to simpeler and more general models, and we would have had a
better understanding of schema evolution and temporal databases.
That is equally interesting to me. To your knowledge has anyone ever
done such a study?
I know of some work done in the context of ORM by Erik Proper in his
PhD thesis: H.A. (Erik) Proper. A Theory for Conceptual Modelling of
Evolving Application Domains. University of Nijmegen, Nijmegen, The
Netherlands, EU, 1994, ISBN 909006849X.
Erik worked closely with Arthur ter Hofstede on the formalization of
ORM. This doesn't make light reading, but it might give an idea of what
I'm talking about.
-- Jan Hidders
.
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