Re: A pk is *both* a physical and a logical object.

JOG <jog_at_cs.nott.ac.uk> wrote:

[snip]

>Yes, pretty much. Lots of possible constructs. Which of the books are
>the same?
>
>1) All - all harry potter and the deathly hallows (identifying
>attribue for a "book" - title)
>2) None - all the copies are different (identifying attribute for a
>"book" - barcode)
>3) The two paperpack versions (identifying attribute for a "book" -
>isbn)
>4) All the english versions (identifying attribute for a "book" - its
>content)
>5) All the english versions without illustrations (identifying
>attribute for a "book" - its text)
>6) etc, etc...

etc. 1) Lightly edited versions for different markets (e.g. U.S. vs. U.K. spelling).

etc. 2) Different titles: "Harry Potter and the Philosopher's Stone" and "Harry Potter and the Sorcerer's Stone".

>All are valid answers. No context to the question - no suitable
>answer. Pick the wrong one for the context you need, broken schema.

     Mine, too. A nasty mess.

>Here we are comparing different items, but we could just as easily be
>comparing the things at different points in time. Something is only
>the same entity if /for the context we chose/ its identifying
>attribute is the same - all of its other properties may change, but if
>the identifying attribute changes then it is a different thing as far
>as that context is concerned.
>
>Again let me emphasize that this is all at the conceptual level. But
>it is only when one has that level sorted that one can move down to
>the logical encoding.

[snip]

Sincerely,

Gene Wirchenko

Computerese Irregular Verb Conjugation:

     I have preferences.
     You have biases.
     He/She has prejudices.