Re: One-To-One Relationships

On Dec 5, 7:09 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
> On 5 dec, 15:45, vldm10 <vld..._at_yahoo.com> wrote:
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> > On Dec 4, 5:20 pm, Jan Hidders <hidd..._at_gmail.com> wrote:
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> > > On 4 dec, 21:55, vldm10 <vld..._at_yahoo.com> wrote:
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> > > > On Dec 3, 6:01 am, Jan Hidders <hidd..._at_gmail.com> wrote:
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> > > > > On 2 dec, 18:53, vldm10 <vld..._at_yahoo.com> wrote:
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> > > > > > On Dec 2, 4:36 am, Jan Hidders <hidd..._at_gmail.com> wrote:
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> > > > > > > On 1 dec, 06:26, vldm10 <vld..._at_yahoo.com> wrote:
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> > > > > > > > On Nov 30, 9:34 am, Jan Hidders <hidd..._at_gmail.com> wrote:
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> > > > > > > > > Why by only one attribute? Why not by a set of attributes? Or a
> > > > > > > > > combination of attributes and relationships (as is the case for weak
> > > > > > > > > entities)?
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> > > > > > > > This is OK, but my advice to you -don't use it often.
> > > > > > > > I will give you one example:
> > > > > > > > The relation has A1, A2, A3, A4 "attributes" and they are mutually
> > > > > > > > independent (i.e. they are in BCNF)
> > > > > > > > The "attributes" can change their values for "entity" like in
> > > > > > > > "temporal DB". User needs on line all information for any "entity" in
> > > > > > > > any moment.
> > > > > > > > Can you please write the key for this relation so that we can discuss
> > > > > > > > it.
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> > > > > > > You do realize we were talking about ER modeling, not RM modeling,
> > > > > > > don't you?
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> > > > > > Here in this tread it is about E/R and RM as well as relationship
> > > > > > among them and I also used terms "entity" and "attribute".
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> > > > > My remark that you responded to was only about ER modelling.
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> > > > I tried to explain my answer through an example.
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> > > You formulated the explanation of your answer to an issue in ER
> > > modeling in RM terminology. If you would have formulated it in ER
> > > terminology that might have helped you considerably to make your
> > > point. RIght now I still don't have a clue what you are trying to tell
> > > me or even whether it is actually relevant for the question at hand.
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> > > -- Jan Hidders- Hide quoted text -
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> > > - Show quoted text -
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> > Jim wrote "...... and identify them (entities) by one attribute..." and
> > you asked "why by only one attribute? Why not by a set of
> > attributes?"
> > My objection here is related to misunderstanding what is identifying?
> > What is construction to make entities to be distinct? What we use for
> > identifying? What is difference between "distinguishing" and
> > identifying? etc
> > On your question "Why not by a set of attributes?" we not identify an
> > entity I gave the answer: "This is OK, but my advice to you - don't
> > use it often."
> > To get clue I will gave you one example:
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> > During a phone call, we would never say the following: "May I speak
> > with the 5 foot tall and has blue eyes and has brown hair and ..."
> > Rather, we will say: "May I speak with John?"
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> > I wrote small theory about identifying and above example is from
> > there. I already recommended you to read it (if you are interested?)
> > You can find it on my website: www.dbdesign10.com
> > look under the following: "7.1 Identifying the Plurality"
> > I am not interested in further discussion about this.
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> *shrug* Suit yourself.
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> -- Jan Hidders- Hide quoted text -
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> - Show quoted text -

I must tell you that it is not possible to identify an entity by "a set of attributes? Or a combination of attribute and relationship (as is the case for weak entities)?
An entity by definition is an object in the real world while a set isn't. An entity's attributes are "physical" while relational's aren't. If you formulate properly your question, then it will be possible to identify an entity by a set of attributes. Act of identifying is specific.
You should be aware what it means to identify an entity which has about 10 attributes on terabyte population. Not to mention identifying with "Temporal DB" or with transfer of data from one environment to another
You can notice that 99.99% of DBs are with one column key.

Vladimir Odrljin Received on Fri Dec 07 2007 - 11:22:58 CET