From: CB
To: Editor
Date: July 9, 2003
I read with great interest the "Double Trouble"
articles on your site.
One question remains, however: what is the proper way to
represent two or more identical objects in a database - say, for example, in an
Orders database where two identical cans of cat food were ordered by a
customer?
My guess is that you would do it one of two ways, as shown
here:
===========================================
ITEM PRICE SEQUENCENO
===========================================
Cat Food 0.39 1
Cat Food 0.39 2
-------------------------------------------
Or:
==================================
ITEM PRICE QUANTITY
==================================
Cat Food 0.39 2
----------------------------------
But I could be wrong.
Please enlighten me.
From: Fabian Pascal
To: CB
Read the chapter on duplicates in my book.
The short answer is that you are confused conceptually; there
IS no such thing as "two identical objects". If they were identical,
they you would not be able to distinguish between, or count them. The fact that
you can means that there is something that distinguishes them and if you want
to keep track of them individually, you MUST represent that distinction in the
database. If you do not, you are contradicting yourself.
From: CB
Thanks. I will read
your book when I get a chance.
But on a practical basis, how would you represent the fact
that a customer purchased 2 cans of cat food on a given date, as a part of one
order?
From: Fabian Pascal
You have only two options, anything else confuses the issue
and introduces inconsistency.
·
either you want to track them individually, in which
case you must distinguish between them, so you use a surrogate key;
·
or you don't need to track them individually, then you
record the lot with an attribute QUANTITY=2;
This is explained in detail in the book – reading the book is
the practical basis.
Posted
09/26/03
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