Re: Examples of SQL anomalies?

From: Brian Selzer <brian_at_selzer-software.com>
Date: Sat, 28 Jun 2008 23:51:47 -0400
Message-ID: <nFD9k.5753$LG4.2422@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.

> In SQL as in Set Theory, equality (=) and grouping are not the same;
> the SUM() and the + are not the same. They are for different levels
> of abstraction. It makes senses after your first course with
> transfinite numbers -- the cardinality of Aleph Null is not the same
> as counting all the integers one by one, etc.
Received on Sat Jun 28 2008 - 22:51:47 CDT

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