"Mikito Harakiri" <mikharakiri_at_ywho.com> wrote in message
news:irT2a.10$O%2.40_at_news.oracle.com...
> "Lauri Pietarinen" <lauri.pietarinen_at_atbusiness.com> wrote in message
> news:3E4B8137.2080204_at_atbusiness.com...
> > < quotes from book Hector Garcia-Molina, Jeffrey D. Ullman, and
> > Jennifer Widom, DATABASE SYSTEM IMPLEMENTATION>
> >
> > [Relational] algebra was originally defined as if relations were sets
> > [sic!--italics added].Yet relations in SQL are really bags ... Thus, we
> > shall introduce relational algebra as an algebra on bags.
> >
> > ...
> >
> > For instance, you may have learned set-theoretic laws such as A
> > INTERSECT (B UNION C) = (A INTERSECT B) UNION (A INTERSECT C), which is
> > formally the "distributive law of intersection over union." This law
> > holds for sets, but not for bags.
> >
> > < quotes from book/ >
>
> Therefore, the idea here is that Set Algebra is superior to Bag Algebra?
Not
> for aggregates:
>
> PROJECTION*AGGREGATE != AGGREGATE*PROJECTION
>
> for example
>
> select distinct S from (
> select distinct SUM(SAL) from emp
> )
>
> is not the same as
>
> select distinct SUM(SAL) from (
> select distinct SAL from emp
> )
>
> where i'm using SQL with the "distinct" keyword merely as a surrogate for
> true relational syntax ("distinct" is redundant after aggregate operation,
> of course).
Why should the sum of all salaries equal the sum of distinct salaries?
Received on Thu Feb 13 2003 - 17:47:32 CST
