Re: Does entity integrity imply entity identity?
Date: Thu, 06 Aug 2009 11:28:05 GMT
"Mr. Scott" <do_not_reply_at_noone.com> wrote in message
> "paul c" <toledobythesea_at_oohay.ac> wrote in message
>> Mr. Scott wrote:
>>> If the requirement is to record in the database as much information as
>>> is available, ...
>> Imaginary requirements are rampant. Dattabases should record only the
>> information that is necessary for their individual purposes. Everything
>> else is wasted so-called econony. In most enterprises, the supposed
>> purpose is usually fake.;
> Are you trying to say that there should never be any missing information?
In theory, there is no difference between theory and practice. In practice, there is.
Every information system needs a way of marking the absence of data in a place where there might have been data. In some situations, there not only might have been data, but there should have been data. In other situations, a place for data has been created, but it is inapplicable to the situation.
In an ideal world, information is never missing if it isn't supposed to be missing. In the real world, a system can be behave somewhat more robustly in the face of missing information if it has a systematic way of dealing with missing information. In a table, the number of places for data is always the product of the order and the cardinality. Depending on table composition, this can result in places for data where there is no data. SQL uses NULL to mark such an occurence. The behavior of the SQL evaluator in the face of NULLs is sometimes surprising, and less than ideal.
I don't think I'm telling you anything you don't already know. Received on Thu Aug 06 2009 - 13:28:05 CEST