Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!postnews.google.com!g14g2000cwa.googlegroups.com!not-for-mail From: "Marshall Spight" Newsgroups: comp.databases.theory Subject: Re: Types and "join compatibility" Date: 8 Aug 2005 00:53:33 -0700 Organization: http://groups.google.com Lines: 65 Message-ID: <1123487612.981221.298680@g14g2000cwa.googlegroups.com> References: <1122744264.923970.145560@f14g2000cwb.googlegroups.com> <1122761293.933576.29120@o13g2000cwo.googlegroups.com> <1123181919.455069.151790@g49g2000cwa.googlegroups.com> NNTP-Posting-Host: 24.4.95.46 Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable X-Trace: posting.google.com 1123487618 14986 127.0.0.1 (8 Aug 2005 07:53:38 GMT) X-Complaints-To: groups-abuse@google.com NNTP-Posting-Date: Mon, 8 Aug 2005 07:53:38 +0000 (UTC) In-Reply-To: User-Agent: G2/0.2 Complaints-To: groups-abuse@google.com Injection-Info: g14g2000cwa.googlegroups.com; posting-host=24.4.95.46; posting-account=s2xCFw0AAAD2mIwYYHAqjdsecwG0axmW Xref: dp-news.maxwell.syr.edu comp.databases.theory:32745 x wrote: > "Andr=E9 Ncss" wrote in message > news:1123181919.455069.151790@g49g2000cwa.googlegroups.com... > > >But as I thought about this I realized that it would be much simpler > >to consider the basic operators. We should really just have to > >consider union, difference, restrict, project and product. Of these, > >the only one that requires us to think about types is clearly > >union. > > And difference and restrict. Okay, difference I can see: what if the subtrahend has attributes that are not in the minuend? But that's not much of an issue; you either say 1) it doesn't matter; ignore it or 2) not allowed. It hardly matters which one you pick. But restrict? It's kind of weird in that it takes a relation and a predicate, (instead of two relations) but the result type is always the same as the relation. So what's the issue? > >One thing that troubles me is the > >relationship between types and constraints. > >To me it seems that types > >and constraints overlap to a large degree. I propose they are completely distinct: types are checked only at compile time; constraints are checked only at runtime. Types can be checked without needing to look at values; constraints must be checked against runtime values. > One thing that troubles me is the > relationship between types and relationships. > > A mathematical relation is a subset of a cartesian product of some sets. > As a set it can be defined by : > a) enumeration > b) properties > > A database relation can be defined by: > a) the system > b) enumeration > c) relational expression > > A TTM scalar type can be defined by > a) the system > b) union of some types - union type > c) cartesian product of some types > d) restriction on a cartesian product of some types > > The case d) define a subset of a cartesian product of some sets which by > definition is a relation (not a relvar). > > What is the active domain in the d) case ? Relation. Maybe I'm not understanding the question. Marshall