Re: Planning aggregates which require sorted or distinct

On Sat, 20 Jan 2007, Tom Lane wrote:

> Gavin Sherry <swm@alcove.com.au> writes:
> > We want to answer the following: for each employee: what is their rank in
> > terms of salary and what is their rank in terms of age. This query
> > answers that:
>
> > select empno, rank() over (order by salary) as srank,
> >   rank() over (order by age) as arank
> >   from employees order by empno;
>
> Eeek.  This seems like the worst sort of action-at-a-distance.  How does
> rank() know what value it's supposed to report the rank of?

This is a frustratingly inconsistent bit of the spec. Rank is defined as
follows:

RANK() OVER WNS is equivalent to:  ( COUNT (*) OVER (WNS1 RANGE UNBOUNDED PRECEDING)  - COUNT (*) OVER (WNS1 RANGE
CURRENTROW) + 1 )
 

Say the salary column has the following values: {100, 200, 200, 300}. This
would give the following output: {1, 2, 2, 4}. DENSE_RANK() would give:
{1, 2, 2, 3}.

These functions are pretty ugly (if you think about them in terms of our
existing aggregates). However, they are by far the most heavily used
window functions (along with ROW_NUMBER()).

Thanks,


Gavin