Re: Does entity integrity imply entity identity?

"Mr. Scott" <do_not_reply_at_noone.com> wrote in message news:BPadnZQTbZN6EefXnZ2dnUVZ_tadnZ2d_at_giganews.com...
>
> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
> news:4a7a425c$0$23748$9a566e8b_at_news.aliant.net...
>> Mr. Scott wrote:
>>
>>> "Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>>> news:4a7a22e9$0$23782$9a566e8b_at_news.aliant.net...
>>>
>>>>Bob Badour wrote:
>>>>
>>>>
>>>>>Mr. Scott wrote:
>>>>>
>>>>>
>>>>>>"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>>>>>>news:4a799414$0$23781$9a566e8b_at_news.aliant.net...
>>>>>>
>>>>>>
>>>>>>>Mr. Scott wrote:
>>>>>>>
>>>>>>>
>>>>>>>>"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message
>>>>>>>>news:4a784836$0$23766$9a566e8b_at_news.aliant.net...
>>>>>>>>
>>>>>>>>
>>>>>>>>>Mr. Scott wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>>The entity integrity rule is generally presented as a subrule
>>>>>>>>>>>under Codd's rule 10.
>>>>>>>>>
>>>>>>>>>I don't know why you would say that when the description of the
>>>>>>>>>so-called "entity integrity" basically restates Rule 2 verbatim.
>>>>>>>>
>>>>>>>>I don't think that's true. Although the entity integrity rule
>>>>>>>>implies the guaranteed access rule, the guaranteed access rule
>>>>>>>>doesn't imply the entity integrity rule. The guaranteed access rule
>>>>>>>>doesn't say anything at all about nulls.
>>>>>>>
>>>>>>>As Martha would say: This is a good thing. Null was a bad idea in the
>>>>>>>first place.
>>>>>>
>>>>>>Why was it a bad idea?
>>>>>
>>>>>Entire books have been written on that subject. I suggest you read one
>>>>>or two of them. See for example Date's various _Writings..._ books.
>>>>>
>>>>>
>>>>>
>>>>>>Is there a reasonable alternative?
>>>>>
>>>>>Yes. Nothing at all is a reasonable alternative to anything as bad as
>>>>>null. If it ain't broken, don't try to fix it. If it is broken, don't
>>>>>break it worse.
>>>>
>>>>Perhaps I should clarify: Your question amounts to asking if one has a
>>>>reasonable alternative to poking oneself in the eye with an icepick.
>>>
>>> If the requirement is to record in the database as much information as
>>> is available, then how can the fact that a value is at present unknown
>>> be recorded?
>>
>> By stating so in the database. That's what relations do: make statements.
>
> How, exactly? If you have columns K, A, B, and C such that for a given K
> it is permissible for the values for A, B or C to be at present unknown,
> but not both A and B, how would you lay out the tables and referential
> constraints without using nulls?

Here's a more concrete example. Software can be delivered to a customer over the internet or through the mail or both, but in order to deliver over the internet there must be an e-mail address, and in order to deliver through the mail there must be a snail-mail address. There may also be a delivery contact phone number. So for a given order K, there must be at least one of an e-mail address A or a snail-mail address B, and there may be a delivery phone number C. Received on Thu Aug 06 2009 - 14:42:29 CEST