Re: NULLs: theoretical problems?

From: Bob Badour <>
Date: Fri, 17 Aug 2007 17:34:15 -0300
Message-ID: <46c60605$0$4034$>

David Cressey wrote:

> "David Portas" <> wrote in message

>>"Hugo Kornelis" <> wrote in message
>>>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
>>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

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