Re: HI
- From: Wolfgang Breitling <breitliw@xxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 20 Sep 2004 12:44:51 -0600
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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