Re: Idempotence and "Replication Insensitivity" are equivalent ?

Chris Smith ha scritto:

> It just means that you form a new relation which contains a subset of
> the information in the first relation by choosing some of the columns.
> If you have an n-ary relation of the form A1 x A2 x A3 x ... x An, then
> there are 2^n - 1 possible projections (excluding the project that
> selects no columns, because it's useless; but quite arbitrarily
> including the identity projection, which is just the original relation).
> Because a relation is a set, the projection will combine any tuples that
> have duplicate values in ALL of the projected columns. So if you have:
>
> Name Age Salary
> ===================================
> Brenda 17 33,000
> Mike 23 27,000
> Bob 45 33,000
> Joe 17 50,000
> Carol 23 27,000
>
...

Thank you for taking the time to teach me that. It's nice. Actually, from the scary name I expected something more complicate ;), but perhaps it's because you explained well :)

It seems that the "Projection" is just the same thing of (a basic instance of what I call) a "Report". It's actually a "vertical" report where all fields chosen play the role of "dimensions" (as opposite to "measures", where instead the values are aggregated through an "aggregate function").

It's the most simple report one can program. It has no measures (hence no totals, no cross measures), no interruptions, no sections: just a simple combination of values of the column "dimensions". Usually you can obtain it just filtering the rows through an sorted list and a "compound comparer" that combines all the comparers defined on the single dimensions of the report. For instance in real reporting you could define one field as case sensitive and another not case sensitive, so the concept of "duplicated row" gets generalized wrt to the common meaning.

Hmmm ... and where this notion of projection is comes into play for dealing with aggregate functions ?

-P

>
> --
> Chris Smith
Received on Thu Sep 21 2006 - 23:16:14 CEST