Re: Examples of SQL anomalies?

From: Brian Selzer <brian_at_selzer-software.com>
Date: Fri, 4 Jul 2008 19:00:15 -0400
Message-ID: <3Yxbk.21319$co7.18979_at_nlpi066.nbdc.sbc.com>


"David Cressey" <cressey73_at_verizon.net> wrote in message news:jSwbk.249$rb1.205_at_trndny08...
>
> "Brian Selzer" <brian_at_selzer-software.com> wrote in message
> news:zOvbk.31378$ZE5.24693_at_nlpi061.nbdc.sbc.com...
>>
>> "David Cressey" <cressey73_at_verizon.net> wrote in message
>> news:D8ubk.240$rb1.211_at_trndny08...
>> >
>> > "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> > news:gI6bk.11000$cW3.8591_at_nlpi064.nbdc.sbc.com...
>> >>
>> >> "David Cressey" <cressey73_at_verizon.net> wrote in message
>> >> news:wq4bk.28$0V1.10_at_trndny01...
>> >> >
>> >> > "Brian Selzer" <brian_at_selzer-software.com> wrote in message
>> >> > news:nFD9k.5753$LG4.2422_at_nlpi065.nbdc.sbc.com...
>> >> >>
>> >> >> "-CELKO-" <jcelko212_at_earthlink.net> wrote in message
>> >> >>
>> >
> news:f219a6bd-9d8e-4cfe-9d60-ce9dcaeff16d_at_z66g2000hsc.googlegroups.com...
>> >> >> >>> The question is, if these issues are due to the SQL
> specification
>> > or
>> >> >> >>> simply due to a problem in a specific SQL product. Or could it
> be,
>> >> > that
>> >> >> >>> the definition is not precise enough in some points, so that
>> > database
>> >> >> >>> vendors implemented it differently? <<
>> >> >> >
>> >> >> > Nope, it is the specs. All aggregate (set) functions begin by
>> >> >> > removing the NULLs from their parameter set, then if there is a
>> >> >> > DISTINCT option on the parameter, they remove redundant
>> >> >> > duplicates
>> > and
>> >> >> > finally do the operation (MIN, MAX, AVG, SUM, COUNT on what is
> left.
>> >> >> > Since an empty set has no elements upon which to apply an
> operation,
>> >> >> > SQL returns a NULL (okay, it should be an "undefined" if we were
>> >> >> > mathematically correct).
>> >> >> >
>> >> >>
>> >> >> MIN, MAX and AVG are meaningless when applied to an empty bag, but
> it
>> >> > seems
>> >> >> to me that COUNT should always return 0 when the bag is empty, and
>> >> >> similarly, SUM should return 0. SUM should only return NULL if one
> of
>> >> >> the
>> >> >> values to be summed is NULL.
>> >> >>
>> >> >
>> >> > By definition, none of the values to be summed are NULL.
>> >> >
>> >>
>> >> OK. OK. Yes, I know. NULL is not a value. I guess to be precise, I
>> > should
>> >> have said "if the cardinality of the set of rows targeted by the query
> is
>> >> greater than the number of values to be summed, then SUM should return
>> >> NULL," but I think that would have caused more confusion. Bottom
>> >> line:
>> >> if
>> >> not all of the amounts are known, then the total amount is suspect.
>> >>
>> >
>> > This has to do with a continuing disagreement between you and me. In
>> > my
>> > view there is no such thing as the "set of rows targeted by the query".
>> > There is a set of data targeted by the query. That is all. Rows that
> do
>> > not contain any data with regard to the current query are not part of
> the
>> > target, by definition.
>> >
>> > Thus, sum (SALARY) from FUBAR is the sum of a bag of salaries. It's
> not
>> > the sum of a bag of rows from FUBAR. Rows in FUBAR that don't contain
>> > a
>> > SALARY are not part of the sum. That is all.
>> >
>>
>> What about sum (SALARY) from FUBAR where Fu = 'Bar'? Doesn't "where Fu =
>> 'Bar'" restrict the query to the result of evaluating "where Fu = 'Bar'"?
>> Clearly if COUNT(*) where Fu = 'Bar' is greater than COUNT(*) where Fu =
>> 'Bar' and SALARY is not NULL, then it is obvious that some salaries
>> aren't
>> known, hence the total of those salaries, sum (SALARY) where Fu = 'Bar'
>> is
>> suspect.
>>
>> I can't understand that you don't get this since it is just so simple.
>>
>
> It is simple, I do get it, and you are wrong.
>

How can that be? If I have ten packages that need to be shipped, but I only know what eight of them weigh, then how can I print a bill of lading that requires the total weight for the shipment? Obviously, the sum of just the eight weights will be less than what is actually shipped, so it should be obvious that any attempt to pass that sum off as the total weight must be held suspect. Received on Sat Jul 05 2008 - 01:00:15 CEST

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