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Home -> Community -> Usenet -> comp.databases.theory -> Re: completeness of the relational lattice
On Jun 22, 9:42 am, Jan Hidders <hidd..._at_gmail.com> wrote:
> Marshall schreef: > > Any expression of the form > > > E \/ [x] > > > can be rewritten as > > > E \/ (X /\ 00) > > > where X is any relation with the same header as [x]. > > ?? Either you can express it or you cannot. Either you can give me an > expression equivalent to [x] or you cannot. An expression that is only > equivalent under certain circumstances is not good enough. It means > your expressive power in terms of queries that you can express drops > below that of the unions of conjunctive queries (UCQ). If the database > consists of just R(a,b) then how can I project on {a}, or on {c,d}?
You cannot. If the database consists of just a single nonempty relation R(x,y), then the whole lattice consists just of 4 elements:
00, R(x,y),
00 /\ R(x,y) = 10, and
00 \/ R(x,y) = 01
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(drumroll)
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For all practical purposes here R(x,y) is indistinguisheble from 11.
Received on Fri Jun 22 2007 - 12:14:56 CDT