Re: WWW/Internet 2009: 2nd CFP until 21 September x
Date: Sat, 15 Aug 2009 20:11:33 GMT
Message-ID: <VDEhm.2272$Jg.1542_at_nwrddc01.gnilink.net>
"paul c" <toledobythesea_at_oohay.ac> wrote in message
news:n1Dhm.41413$PH1.3194_at_edtnps82...
> Walter Mitty wrote:
>> ....
>> The way I think of it is that every table with nulls in it is a
>> materialized outer join. If you can decompose the table into multiple
>> tables each of which has no nulls, what you discover is that a null in
>> the combined table corresponds to an absent row in one of the decomposed
>> tables.
>>
>> Let me shift gears back into practical mode for a minute. In any
>> database I've ever worked with, the majority of columns are not a primary
>> key, or a foreign key or a part of a primary or foreign key, or ever
>> appear in a where of having clause. Nulls in those columns are of almost
>> no consequence at all in the overall behavior of queries. Shunning nulls
>> in those cases is being overly picky.
>>
>> Nulls in "important" columns almost always cause more trouble than
>> decomposing tables would cause, but nulls in inumportant columns help
>> keep things simple.
> ...
>
> An implicit suggestion here is that there is a way to determine which apps
> are 'simple' in some sense, eg., not capable of contradictions, other than
> by avoiding nulls. I don't mind people advocating nulls as long as they
> don't pretend they have a theory, logic and algebra that makes their use
> contradiction-free. If they want their cake and eat it too, they could
> get to work figuring out SQL might prevent the definitions and statements
> that cause apps to be 'non-simple' in some useful sense.
>
I don't so much advocate nulls as that I don't avoid their use entirely. There are some places where missing data does no harm, or at least very little harm.
As far as having a theory goes, I don't think we should go so far as to generate an algebra of missing data. I'm perfectly content with a mathematical base that stipulates that the data we are given is all we need to compute with. But information systems are not just methematics. When Codd said that a DBMS should have a systematic method of dealing with missing data, I think I know what he meant. And I agree. Whether SQL's method is systematic enough is something that could be argued either way. I'm willing to live with the SQL implementation, but I wouldn't want to try to defend it. I'm sure a better job can be done.
>
Received on Sat Aug 15 2009 - 22:11:33 CEST