Re: NULLs: theoretical problems?
From: Bob Badour <bbadour_at_pei.sympatico.ca>
Date: Fri, 17 Aug 2007 17:34:15 -0300
Message-ID: <46c60605$0$4034$9a566e8b_at_news.aliant.net>
>>"Hugo Kornelis" <hugo_at_perFact.REMOVETHIS.info.INVALID> wrote in message
>>news:3vf9c351l07qa5jf0mui4lmccp8oetkonr_at_4ax.com...
>>
>><snip>
>>
>>>
>>>Back to databases. A DB can't store a dash symbol for a missing
>>>proposition, since (a) a dash symbol is not part of each domain, and (b)
>>>the dash symbol might have a special meaning in a column (as in the
>>>rating column mentioned above). Instead, a DB uses NULL as a symbol with
>>>the same meaning aas the dash in ink-and-paper tables: "this space left
>>>empty intentionally".
>>>
>>>The DB table storing the above information would look like this:
>>>
>>> SSN | Name | DOB
>>>-------+---------------+------------
>>> 12345 | John Brown | 1960-08-31
>>> 90163 | Kate Wilson | NULL
>>> 55202 | Geoff T. Hurt | 1973-01-12
>>>
>>>and this is a faithful representation of these five true propositions:
>>>
>>>"The person identified by SSN 12345 goes by the name John Brown."
>>>"The person identified by SSN 90163 goes by the name Kate Wilson."
>>>"The person identified by SSN 55202 goes by the name Geoff T. Hurt."
>>>
>>>"The person identified by SSN 12345 was born on 1960-08-31."
>>>"The person identified by SSN 55202 was born on 1973-01-12."
>>>
>>>The NULL in the DOB column does not represent a sixth proposition; it
>>>just marks the absence of a proposition involving the SSN 90163 and the
>>>verb "was born on".
>>>
>>>I really fail to see how you can call this concept a "theoretical
>>>abhorrence".
>>
>>If those are the ONLY propositions that you intend to represent with your
>>database then you might have a point. The theoretical abhorrence arises as
>>soon as we try to derive other propositions from such a database. Then
>>three-value logic means that true propositions in the real world are no
>>longer true and false ones are no longer false.
Date: Fri, 17 Aug 2007 17:34:15 -0300
Message-ID: <46c60605$0$4034$9a566e8b_at_news.aliant.net>
David Cressey wrote:
> "David Portas" <REMOVE_BEFORE_REPLYING_dportas_at_acm.org> wrote in message > news:6OCdnV108uG4RFjbnZ2dnUVZ8tyqnZ2d_at_giganews.com... >
>>"Hugo Kornelis" <hugo_at_perFact.REMOVETHIS.info.INVALID> wrote in message
>>news:3vf9c351l07qa5jf0mui4lmccp8oetkonr_at_4ax.com...
>>
>><snip>
>>
>>>
>>>Back to databases. A DB can't store a dash symbol for a missing
>>>proposition, since (a) a dash symbol is not part of each domain, and (b)
>>>the dash symbol might have a special meaning in a column (as in the
>>>rating column mentioned above). Instead, a DB uses NULL as a symbol with
>>>the same meaning aas the dash in ink-and-paper tables: "this space left
>>>empty intentionally".
>>>
>>>The DB table storing the above information would look like this:
>>>
>>> SSN | Name | DOB
>>>-------+---------------+------------
>>> 12345 | John Brown | 1960-08-31
>>> 90163 | Kate Wilson | NULL
>>> 55202 | Geoff T. Hurt | 1973-01-12
>>>
>>>and this is a faithful representation of these five true propositions:
>>>
>>>"The person identified by SSN 12345 goes by the name John Brown."
>>>"The person identified by SSN 90163 goes by the name Kate Wilson."
>>>"The person identified by SSN 55202 goes by the name Geoff T. Hurt."
>>>
>>>"The person identified by SSN 12345 was born on 1960-08-31."
>>>"The person identified by SSN 55202 was born on 1973-01-12."
>>>
>>>The NULL in the DOB column does not represent a sixth proposition; it
>>>just marks the absence of a proposition involving the SSN 90163 and the
>>>verb "was born on".
>>>
>>>I really fail to see how you can call this concept a "theoretical
>>>abhorrence".
>>
>>If those are the ONLY propositions that you intend to represent with your
>>database then you might have a point. The theoretical abhorrence arises as
>>soon as we try to derive other propositions from such a database. Then
>>three-value logic means that true propositions in the real world are no
>>longer true and false ones are no longer false.
> > > In theory, you are right. In practice, it is really quite simple to avoid > queries that can be tripped up by three valued logic. The queries relate to > propositions that are actually stored in the database, and not propositions > that are not stored. > > This is even true for queries that may contain some NULLs in the result > table. Just as well, because when doing outer joins, one would come up > with NULLS in the result table, even if the base tables were designed in > such a way that NULL need never be permitted to be stored.
Since all an outer-join is is a union, why would we care? We can write the union so that it doesn't return any NULLs. Received on Fri Aug 17 2007 - 22:34:15 CEST