Missing Values

Missing Values

Post by ro.. » Wed, 18 Dec 1996 04:00:00



I am designing an object oriented language and have run into one of
the classic problems of database theory, namely the proper handling of
unknown values..

I know of the 3VL ( T, F, ? ) and 4VL ( T, F, ?, N/A ) but these seem
to be inadequate solutions to the problem...

what other approaches have been taken to this problem?

Thanks for any help you can give me :)

 
 
 

Missing Values

Post by Robert P. Biuk-Agh » Thu, 19 Dec 1996 04:00:00



> I am designing an object oriented language and have run into one of
> the classic problems of database theory, namely the proper handling of
> unknown values..

You could consider staying within the framework of two-valued logic and
using default values to represent missing values. For example, where an
existing value might be a positive integer, a default value could be -1,
for a character value, say a name, the default could be "???" or just "".

I recommend you turn to some of C.J. Date's writings for some reasons
why nulls and many-valued (i.e. >2) logic are a bad idea. His "Introduction
to Database Systems", 6th ed., has a whole chapter on the topic (Chapter 20,
"Missing Information"), and his "Selected Writings" books have more coverage
of nulls and missing information.

Hope this helps,

Robert.

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Robert P. Biuk-Aghai,  University of Macau, Faculty of Science and Technology

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     The best beloved of all things in My sight is Justice ...
                                                 - Baha'u'llah (1817-92)

 
 
 

Missing Values

Post by ro.. » Thu, 19 Dec 1996 04:00:00




>> I am designing an object oriented language and have run into one of
>> the classic problems of database theory, namely the proper handling of
>> unknown values..
>You could consider staying within the framework of two-valued logic and
>using default values to represent missing values. For example, where an
>existing value might be a positive integer, a default value could be -1,
>for a character value, say a name, the default could be "???" or just "".

I had considered this approach but i keep hitting the mental block
which tells me that a missing value is not a value and that using a
default value "feels" wrong...  [ Of course I will use defaults where
they are appropriate however :) ]

The problem is with the N/A case where a value is not applicable...
Using a default value is a violation of integrity in this case...

Thank you so much for your advice and the list of readings :)  I'm
gonna pick them up at the library this afternoon :)

 
 
 

Missing Values

Post by Karen A. Morrisse » Sat, 21 Dec 1996 04:00:00



> [...]
> I had considered this approach [using illegal values to indicate
> missing values] but i keep hitting the mental block
> which tells me that a missing value is not a value and that using a
> default value "feels" wrong...  [ Of course I will use defaults where
> they are appropriate however :) ]
> [...]

I've found 3- and 4-valued logic quite sensible, but that may be an
effect of the many years I've worked with Prolog (a logic-based
programming language) as a database manipulation language.

The problem is that SQL doesn't really support missing values.

Paraphrased from an article which appeared about 10 years ago in an ACM
periodical: "If programmers became able to program in logic, we would
discover that programmers are not logical." That year I learned from
direct observation that the writer was correct and optimistic.

--


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Missing Values

Post by Joel Gar » Tue, 24 Dec 1996 04:00:00




>> [...]
>> I had considered this approach [using illegal values to indicate
>> missing values] but i keep hitting the mental block
>> which tells me that a missing value is not a value and that using a
>> default value "feels" wrong...  [ Of course I will use defaults where
>> they are appropriate however :) ]
>> [...]

>I've found 3- and 4-valued logic quite sensible, but that may be an
>effect of the many years I've worked with Prolog (a logic-based
>programming language) as a database manipulation language.

>The problem is that SQL doesn't really support missing values.

>Paraphrased from an article which appeared about 10 years ago in an ACM
>periodical: "If programmers became able to program in logic, we would
>discover that programmers are not logical." That year I learned from
>direct observation that the writer was correct and optimistic.

I would suggest not using any values that would correspond to a clue that
the database is corrupt.  For example, find out what the database thinks
is the value of a space that is misdefined as a number, and don't use that.
This hint was just about useless when database engines controlled all
metadata, but with the advent of metadata distribution and different
ways of accessing archived data these errors are coming back again.

Columns of 8224 was always a clue that something was wrong in the DMS-500
world.  :)

jg

>--


>1801 California          (on assignment from Analysts International)
>Suite 310
>Denver, CO 80202
>303-965-8473

--

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