Path: dp-news.maxwell.syr.edu!spool.maxwell.syr.edu!drn.maxwell.syr.edu!news.maxwell.syr.edu!area.cu.mi.it!news.newsland.it!ilaria.aioe.org!aioe.org!not-for-mail From: "x" Newsgroups: comp.databases.theory Subject: Re: Types and "join compatibility" Date: Mon, 8 Aug 2005 10:03:32 +0300 Organization: Aioe.org NNTP Server Lines: 42 Message-ID: References: <1122744264.923970.145560@f14g2000cwb.googlegroups.com> <1122761293.933576.29120@o13g2000cwo.googlegroups.com> <1123181919.455069.151790@g49g2000cwa.googlegroups.com> NNTP-Posting-Host: AeEOSkNYeT2BnqGmDsT9eQ.domitilla.aioe.org X-Complaints-To: abuse@aioe.org NNTP-Posting-Date: Mon, 8 Aug 2005 07:03:32 +0000 (UTC) X-Newsreader: Microsoft Outlook Express 6.00.2800.1437 Xref: dp-news.maxwell.syr.edu comp.databases.theory:32743 "André 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. >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. 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 ?