[AccessD] 2 quick questions

Asger Blond ab-mi at post3.tele.dk
Mon May 30 20:26:20 CDT 2011


Jim,
I don't want to reiterate this discussion but your comment puzzles me.
I quite agree that an autonumber is not a primary key, but I would then hasten to specify that it is not so in the *logical design* - in the *physical implementation* it certainly could (and IMO should) be.
So just to get you right I would like to know:
Do you actually implement the natural key as a primary key and then create a unique index on the autonumber field? And also: when implementing a foreign key, do you make this key point to the natural key rather than to the autonumber key?
If your answer is yes to any of these questions, then I disagree and we have a point of discussion.
If not, I don't get the meaning in your comment. Then you are in accordance with Robert, and I suggest with most of us.
The disagreement appears to me to be a matter of wording, or if you prefer: definition - neither of which are trivial! And distinguishing a *logical PK* and a *physical PK* would clarify the discussion. 
Asger

-----Oprindelig meddelelse-----
Fra: accessd-bounces at databaseadvisors.com [mailto:accessd-bounces at databaseadvisors.com] På vegne af Jim Dettman
Sendt: 30. maj 2011 02:19
Til: 'Access Developers discussion and problem solving'
Emne: Re: [AccessD] 2 quick questions

<<Primary Key = Nothing a user can use for anything and is totally not 
dependant on data. For db system use only>>

  Pure hogwash.  The very definition of a PK in the relational model is
exactly opposite of what you describe.

  John's problem is exactly what I tried to point out on this list a while
back; there is a fundamental difference between a primary key and a tag or a
pointer.  John's problem is related to the meaning of the data itself; how
to determine that a row within the relation is unique.  In other words, John
is looking for the primary key; that combination of attributes which
identifies a specific row and ensures that the row is not duplicated.  

  That is not the same thing as adding a unique tag or pointer on every row
(an auto number).  And despite popular belief, a auto number in relational
terms is not a surrogate key.

  John needs to come up with a key based on the data that will ensure that a
row is unique.  If the field(s) used to do that cannot ensure that, then
more fields (attributes) need to be added to the table (relation).

  For *performance* reasons in the DB, he will use an auto number as a
pointer (like all of us do), but in addition to that, he will need to create
a unique index based on the PK.

  He might take the shortcut of making that index a hash, which for
performance reasons may be required, but it's a bad idea because it can lead
to an update abnormalities.  If you can live with that, great and at the end
of the day, it may be required to get the job done.

  Since it's only one index on the main table, I wouldn't take that
shortcut.

  And as far as the primary key not being able to change (as in a name
change or SS#), again total hogwash.  That's why relational DB's allow for
cascading updates.  There is nothing stated in any paper on the relational
model that says a primary key cannot change.

Jim.
 

-----Original Message-----
From: accessd-bounces at databaseadvisors.com
[mailto:accessd-bounces at databaseadvisors.com] On Behalf Of Robert Stewart
Sent: Friday, May 27, 2011 04:03 PM
To: accessd at databaseadvisors.com
Subject: Re: [AccessD] 2 quick questions

Primary Key = Nothing a user can use for anything and is totally not 
dependant on data. For db system use only

Business Key = Something within the data that is unique and can be 
used by a user for identification.

Business Keys can be one or more fields, when more than one, it is a 
Composite Business Key.

Remember the above and you will not have any issues.

Example:

tlkp_NameSuffix
NameSuffixID   Autonumber
NameSuffix      Text(30)

NameSuffixID is the primary key
NameSuffix is the business key, and has a unique index on it.


Robert L. Stewart
www.WeBeDb.com
www.DBGUIDesign.com
www.RLStewartPhotography.com

At 01:18 PM 5/27/2011, you wrote:
>Date: Fri, 27 May 2011 08:18:47 -0500
>From: "Dan Waters" <df.waters at comcast.net>
>To: "'Access Developers discussion and problem solving'"
>         <accessd at databaseadvisors.com>
>Subject: Re: [AccessD] [Spam]8.31 Re: 2 quick questions
>Message-ID: <001101cc1c70$9d48a380$d7d9ea80$@comcast.net>
>Content-Type: text/plain;       charset="us-ascii"
>
>A field with an 'Intelligent Key' is fine - as long as you don't use it in
a
>table relationship.  In other words, always have an arbitrary (autonumber,
>etc.) key as the key that is used in table relationships.  That way you'll
>NEVER have to spend hours or days rebuilding tables when the key you used
in
>a dozen or more table relationships is changed.
>
>Dan
>
>PS - Even RFID's can be swapped out, and Tattoos can be removed!
>
>-----Original Message-----
>From: accessd-bounces at databaseadvisors.com
>[mailto:accessd-bounces at databaseadvisors.com] On Behalf Of Stuart McLachlan
>Sent: Friday, May 27, 2011 1:06 AM
>To: Access Developers discussion and problem solving
>Subject: Re: [AccessD] [Spam]8.31 Re: 2 quick questions
>
>Intelligent Keys are an excellent concept when you can attach them  to  the
>item concerned or to it's container.
>
>Unless you can tattoo them or implant an RFID chip, there is little point
in
>trying to use IKs to identify people.  :-)
>
>--
>Stuart
>
>On 27 May 2011 at 1:55, Arthur Fuller wrote:
>
> > I don't know where you obtained this rule against the use of
> > Intelligent Keys, but I seriously object, and cite Chris Date and
> > Fabian Pascal and the inestimable Joe Celko for reasons why IKs are a
> > good thing, in certain applications (eggs are a bad example; car parts
> > are an excellent example).
> >
> > A.
> > --
> > AccessD mailing list
> > AccessD at databaseadvisors.com
> > http://databaseadvisors.com/mailman/listinfo/accessd
> > Website: http://www.databaseadvisors.com
> >
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