Re: Expressions versus the value they represent
From: paul c <toledobythesea_at_oohay.ac>
Date: Thu, 15 Apr 2010 02:31:41 GMT
Message-ID: <hUuxn.1831$Z6.1124_at_edtnps82>
>
> You are merely talking about the correspondence between formulae in
> FOL versus expressions in the RA. E.g.
>
> RA: project((R1 join R2) union R3, {X})
> FOL: exists y,z such that (P1(x,y) /\ P2(y,z)) \/ P3(x,y,z)
>
> R1,R2,R3 are relvar names. X is an attribute name. P1,P2,P3 are
> predicate symbols. x,y,z are variables.
> ...
Date: Thu, 15 Apr 2010 02:31:41 GMT
Message-ID: <hUuxn.1831$Z6.1124_at_edtnps82>
David BL wrote:
> On Apr 15, 5:01 am, com..._at_hotmail.com wrote:
...
>> The relational algebra is just another syntax for FOPL. >> (So they can't possibly be at odds.)
>
> You are merely talking about the correspondence between formulae in
> FOL versus expressions in the RA. E.g.
>
> RA: project((R1 join R2) union R3, {X})
> FOL: exists y,z such that (P1(x,y) /\ P2(y,z)) \/ P3(x,y,z)
>
> R1,R2,R3 are relvar names. X is an attribute name. P1,P2,P3 are
> predicate symbols. x,y,z are variables.
> ...
Regarding precision, as far as I know, relvars are pointers and the several relational algebras do not manipulate pointers.
(To me this means that an implementation of such an algebra doesn't require relvars.) Received on Thu Apr 15 2010 - 04:31:41 CEST