Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!border1.nntp.dca.giganews.com!border2.nntp.dca.giganews.com!nntp.giganews.com!cyclone-sf.pbi.net!216.218.192.242!news.he.net!chekhov.conxion.net!news.oracle.com!not-for-mail From: "Mikito Harakiri" Newsgroups: comp.databases.theory References: <3RMQd.35$YQ1.109@news.oracle.com> <4213e7b7$0$53482$ed2619ec@ptn-nntp-reader03.plus.net> <1108605544.024502.230810@l41g2000cwc.googlegroups.com> Subject: Re: What is Aggregation? Re: grouping in tuple relational calculus Lines: 37 X-Priority: 3 X-MSMail-Priority: Normal X-Newsreader: Microsoft Outlook Express 6.00.2600.0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2600.0000 Message-ID: Date: Thu, 17 Feb 2005 16:12:27 -0800 NNTP-Posting-Host: 130.35.179.159 X-Trace: news.oracle.com 1108686355 130.35.179.159 (Thu, 17 Feb 2005 16:25:55 PST) NNTP-Posting-Date: Thu, 17 Feb 2005 16:25:55 PST Xref: dp-news.maxwell.syr.edu comp.databases.theory:30013 "Jan Hidders" wrote in message news:MO9Rd.14260$fH2.1142837@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. When I open any math books, sigmas is the only aggregate I see. Others -- products and lower/upper bounds -- are rarities. > > 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?"