Re: Idempotence and "Replication Insensitivity" are equivalent ?
Date: 24 Sep 2006 07:54:15 -0700
> Aloha Kakuikanu wrote:
> > vc wrote:
> > > According to ANSI SQL, nulls are ignored in the aggregate functions
> > > (except count(*)). It may not make sense to you, but it makes sense
> > > to a lot of other folks.
> > Or yeah, those folks defined null as a neutral element for sum
> > aggregate too. Apparently they are unaware of identity:
> > 0 + 0 + ... = 0
> The above does not make any obvious sense. What's a "neutral element"
There are actually 2 issues:
1. How sum is defined on empty set. It is certainly 0, not null. 2. How the sum is defined when there are unknown elements. Unknown elements are interpreted as 0s.
> > What I'm saying is that you have to define different neutral elements
> > for different aggregates in order to maintain elementary consistency
> Could you, like, you know, rephrase that ?
null != null
And you don't have to go far for the concrete examples, e.g. summing null and x explicitly gives null, while doing this via aggregation gives x! Received on Sun Sep 24 2006 - 16:54:15 CEST