Re: What is Aggregation? Re: grouping in tuple relational calculus
From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Fri, 18 Feb 2005 22:33:15 GMT
Message-ID: <L2uRd.14949$TV7.1119364_at_phobos.telenet-ops.be>
>
> Unless it's a text string aggregation (defined upon binary string
> concatenation operation). String concatenation is not symmetric, and
> string aggregation is defined over lists.
>
>
> Well, the argument that there are potentially infinite numbers of
> aggregate operators isn't really very convincing.
>
> Citing David Cressey: "Can you trust relational language
> specification to the users?"
Date: Fri, 18 Feb 2005 22:33:15 GMT
Message-ID: <L2uRd.14949$TV7.1119364_at_phobos.telenet-ops.be>
Mikito Harakiri wrote:
> "Jan Hidders" <jan.hidders_at_REMOVETHIS.pandora.be> wrote in message
> news:MO9Rd.14260$fH2.1142837_at_phobos.telenet-ops.be...
>
>> Mikito Harakiri wrote: >> >>> So, what is the defining property of an aggregate? >> >> The defining property is that it is a function over bags. So if you >> want to base it on a binary operation this operation has to >> symmetric and associative.
>
> Unless it's a text string aggregation (defined upon binary string
> concatenation operation). String concatenation is not symmetric, and
> string aggregation is defined over lists.
>>> Next, is sum the only aggregate that can't be expressed by >>> standard means?
>
>> Actually there is an infinite number of them, unless you want to >> limit the term artificially to those that are found in a certain >> query language.
>
> Well, the argument that there are potentially infinite numbers of
> aggregate operators isn't really very convincing.
It means that you have to be careful with arguments like "I only know a few and therefore it must be rare". There may be a whole universe out there of stuff that people would like to be able to do and you have never heard about. Looking in math books is not a good way to find out what people typically want to do with their database.
>>> If it is, then don't you think that justification for an >>> aggregation syntax is too thin? >> >> If the users can understand and use it easily and it can be >> implemented efficiently then that is all the justification that is >> needed. Mathematical elegance is only circumstantial evidence here.
>
> Citing David Cressey: "Can you trust relational language
> specification to the users?"
?? I never said we should.
- Jan Hidders