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From: Nilone <reaanb@gmail.com>
Newsgroups: comp.databases.theory
Subject: Re: more on delete from join
Date: Sat, 29 Aug 2009 13:40:42 -0700 (PDT)
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On Aug 29, 5:39=A0am, Kevin Kirkpatrick <kvnkrkpt...@gmail.com> wrote:
> On Aug 28, 6:29=A0pm, Bob Badour <bbad...@pei.sympatico.ca> wrote:
> > But computers do solve systems of equations:
> >
> > x+y =3D 3
> > x-y =3D 1
>
> Well, I will grant that. =A0 But there is an difference between
>
> exp[0]:=3D'x+y =3D 3'
> exp[1]:=3D'x-y =3D 1'
> print(Solve_system (exp, 'x')); -- prints 2
>
> and
> x+y :=3D 3;
> x-y :=3D 1;
> print (x); =A0-- prints 2? (does it? can it? see below...)
>
> To see why these must be treated differently, consider:
>
> exp[0] :=3D 'x=3D4'
> exp[1] :=3D 'x=3D5'
> print(Solve_system (exp, 'x')); -- error!
>
> vs.
>
> x:=3D4;
> x:=3D5; -- no error
> print(x); -- prints 5 (of course)
>
> I simply still see no hole in the following line of reasoning:
> 1) =A0A query is a value. =A0Its value is of type "valid query" - a strin=
g
> that can be evaluated by the DBMS (returning a relation value).
> 2) =A0Views are variables that hold values of type 'valid query'.
> 3) =A0The only sense by which views can be updated is in the sense that
> their values of type "valid query" can be modified. =A0Below are some
> valid view assignments:
> view x, y, z;
> x:=3D 'select 5 c1 from dual';
> y:=3D 'select 6 c1 from dual';
> z :=3D 'select * from x union select * from y';
> 4) As conventionally understood, "updatable views" result in using
> views as the target of a relational assignment. =A0But this is invalid
> no matter how you look at it. =A0If the assignment is of form <view> :=3D
> <relation>, you've got a type violation (views have type "valid
> query", not "relation value"); and if you treat it as <query
> result> :=3D <relation>, you're assigning to a value rather than a
> variable (which is no more coherent than assignment 5 :=3D 2 + 3;). =A0In
> other words, both of these are obviously illegal:
> z :=3D {{c1=3D7},{c1=3D8}};
> {{c1=3D5},{c1=3D6}} :=3D {{c1=3D7},{c1=3D8}};

This is a good post, and I agree with your reasoning.