Re: Has E/R had a negative impact on db?

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. 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.

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, I have just put words in your mouth, but you shouldn't have left a
hole there. :-)

Not a problem. I left the hole there, I expect it.

If you are going to rant about something you despise,
you better define it properly.

This is circuitous as what I am actually 'despising' is the fact that I
have trouble making that definition.


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.


Sure, we can invent them, but
there is never any inherent truth to their boundaries.

When making an ER / ORM / ... some ER dialect ... model one doesn't
identify boundaries, except perhaps when determining part-of
relationhips, so I have no idea what you are talking about here. Until
you further explain this it looks mainly like pseudophilosophical
nonsense to me.

To me the situation get worse when one then adds relationships between
entities. What on earth is the difference between an entity (an
association of attributes) and a relationship (an association of
entities), except for the fact that a relationship is constrained to
being binary.


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.

Does it also bother you that the distinction
between a relation and a foreign-key relationship is not always clear?
Or between a unary relation and a domain?

I'm not sure how these comparisons are relevent, but yes (although I
believe I am wrong to be bothered by it) and no. I am far more
concerened however with the fact that my hair is starting to grey.


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? Given
an E/R relationship can always be restated as an associative entity I'd
appreciate if you could expand on this.


Now Codd grokked this. I am sure he did. That's why the RM has no
'links' and pivots on the information principle.

Codd is not on you side here. The surrogate identifiers qualify as
"links" and why, if he thought the term "entity" meaningless, did he
use it?

Well I am on no side (these are merely musings). AFAIK surrogate
identifiers were not supposed to be seen or manipulated by the user so
these 'links' were internal, so I'm not clear as to your objection
there.

Either way, the point I was making is that his model does not
distinguish relationships and entities. I'd believe this has proven to
be a good thing - but I get the feeling you think there may be an
alternative?


Okay, so for those in the know this isn't an issue and E/R is a useful
tool. But for those not in the know (which appears to be a lot of the
industry) it promotes the fallacy of the Entity/Relationship
distinction, of impenetrable wrappers, and encourages the mindset that
has lead to OODBMS, XML databases, etc.

Gee, James, here I was thinking I was in the know and it turns out that
I couldn't disagree more, so apparently I'm not(!).

No, outside of being perplexed by the time you've spent working with
XML databases, I'd say you were very much in the know. I value your
responses Jan.

I would argue that
it is exactly *because* ER dialects were ignored as a viable
alternative for a DBMS logical model, and consequently got swallowed up
and crushed in the OODBMS avalanche, that the field of database theory
has been held back. 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?


-- Jan Hidders

All best, Jim.

.

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