Re: more on delete from join

From: Nilone <reaanb_at_gmail.com>
Date: Sat, 29 Aug 2009 13:40:42 -0700 (PDT)
Message-ID: <ee6f4aea-a4df-4229-83c9-6426b5c9dc17_at_33g2000vbe.googlegroups.com>


On Aug 29, 5:39 am, Kevin Kirkpatrick <kvnkrkpt..._at_gmail.com> wrote:
> On Aug 28, 6:29 pm, Bob Badour <bbad..._at_pei.sympatico.ca> wrote:
> > But computers do solve systems of equations:
> >
> > x+y = 3
> > x-y = 1
>
> Well, I will grant that.   But there is an difference between
>
> exp[0]:='x+y = 3'
> exp[1]:='x-y = 1'
> print(Solve_system (exp, 'x')); -- prints 2
>
> and
> x+y := 3;
> x-y := 1;
> print (x);  -- prints 2? (does it? can it? see below...)
>
> To see why these must be treated differently, consider:
>
> exp[0] := 'x=4'
> exp[1] := 'x=5'
> print(Solve_system (exp, 'x')); -- error!
>
> vs.
>
> x:=4;
> x:=5; -- no error
> print(x); -- prints 5 (of course)
>
> I simply still see no hole in the following line of reasoning:
> 1)  A query is a value.  Its value is of type "valid query" - a string
> that can be evaluated by the DBMS (returning a relation value).
> 2)  Views are variables that hold values of type 'valid query'.
> 3)  The only sense by which views can be updated is in the sense that
> their values of type "valid query" can be modified.  Below are some
> valid view assignments:
> view x, y, z;
> x:= 'select 5 c1 from dual';
> y:= 'select 6 c1 from dual';
> z := 'select * from x union select * from y';
> 4) As conventionally understood, "updatable views" result in using
> views as the target of a relational assignment.  But this is invalid
> no matter how you look at it.  If the assignment is of form <view> :=
> <relation>, you've got a type violation (views have type "valid
> query", not "relation value"); and if you treat it as <query
> result> := <relation>, you're assigning to a value rather than a
> variable (which is no more coherent than assignment 5 := 2 + 3;).  In
> other words, both of these are obviously illegal:
> z := {{c1=7},{c1=8}};
> {{c1=5},{c1=6}} := {{c1=7},{c1=8}};

This is a good post, and I agree with your reasoning. Received on Sat Aug 29 2009 - 22:40:42 CEST

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