RE: HI
- From: "Lex de Haan" <lex.de.haan@xxxxxxxxxxxxxx>
- To: <breitliw@xxxxxxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 20 Sep 2004 21:03:50 +0200
the crux in the subquery madness debate was that SQL is missing
functionality.
what we have in mathematics is the concept of composition. For example,
f(g(x)) means apply function g on x *first* and then apply function f on the
result.
f(g(x)) is not the same as g(f(x)) in general.
this is elegant maths syntax to express precisely what you want to happen;
in the subquery madness example, such syntax could make sure that you
*first*
filter column values *before* you try to apply a TO_NUMBER conversion.
however, SQL doesn't make this clear distinction
aaarrrggh, there you go -- it was *not* my intention to open that debate
again...
the operators from the relational algebra (like the restriction example,
given below)
indeed offer full freedom in their processing order.
Kind regards,
Lex.
-------------------------------
visit http://www.naturaljoin.nl <http://www.naturaljoin.nl>
-------------------------------
skype me <callto://lexdehaan>
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx]On Behalf Of Wolfgang Breitling
Sent: Monday, September 20, 2004 19:45
To: oracle-l@xxxxxxxxxxxxx
Subject: Re: HI
I thought Oracle (and db2 and mssql) are relational dbs (with some object
features veneered on). The crux as I see it is that the SQL language is not
built on as rigorous a foundation as the relational algebra and as such let
quite a bit of ambiguity creep in.
Rethinking my statement above, you may be right that they all are not
strictly relational DBs in that relational algebra does not have a concept
of duplicates. Every tuple is uniquely identifiable.
Just out of curiosity I dug out the course material of a course on Database
Design by "The Relational Institute". The material has "copyright 1987 Codd
& Date consulting group" on every page. In the chapter on Query
Optimization under the heading "convert into canonical form" it lists
several transformation rules:
- (A join B) where A-restriction and B-restriction ==> (A where
A-restriction) join (B where B-restriction)
- any restriction predicate ==> equivalent predicate in conjunctive NF
e.g. P or (Q and R) ==> (P or Q) and (P or R)
- (A where restriction 1) where restriction 2 ==> (A where restriction-1
AND restriction-2)
I love this particular one the most. Isn't that exactly the case where
he was arguing in response to Jonathan's subquery madness article that the
subquery must be evaluated before the outer restriction is applied???
- (A [projection-1]) [projection-2] ==> (A [projection-2])
- (A [projection]) where restriction ==> (A restriction) [projection]
- A.F1 > B.F2 and B.F2 = 3 ==> A.F1 > 3 (possibly allows the elimination
of a join!!)
- others
At 12:15 PM 9/20/2004, you wrote:
>You probably are correct Wolfgang, but I thought Oracle is NOT a
>relational db, but an
>SQL db, and if I understand Date correctly, then this qst has relevance.
>
>--
>Rob Zijlstra
Regards
Wolfgang Breitling
Centrex Consulting Corporation
http://www.centrexcc.com
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