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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