Re: proposal: window function - change_number

On Sun, Sep 21, 2014 at 9:27 PM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
wrote:

> Hi
> I tried to solve following task:
>
> I have a table
>
> start, reason, km
> =============
> 2014-01-01 08:00:00, private, 10
> 2014-01-01 09:00:00, commerc, 20
> 2014-01-01 10:00:00, commerc, 20
> 2014-01-01 11:00:00, private, 8
>
> and I would reduce these rows to
>
> 2014-01-01 08:00:00, private, 10
> 2014-01-01 09:00:00, commerc, 20 + 20 = 40
> 2014-01-01 11:00:00, private, 8
>
> It is relative hard to it now with SQL only. But we can simplify this task
> with window function that returns number of change in some column. Then
> this task can be solved by
>
> select min(start), min(reason), sum(km)
> from (select start, reason, km, change_number(reason) over (order by
> start))
> group by change_number;
>
>
I guess that might be quite useful, otherwise the only way that comes to
mind to do this would be something along the lines of:

select *,sum(case when reason <> lastreason then 1 else 0 end) over (order
by start) as chg_num from (select *,lag(reason) over (order by start) vnext
from sometable) sometable;

This way might not be too bad as I think the outer window will have no need
to perform another sort, since the inner window clause has sorted it the
right way already. Though something like change_number() would make this a
bit more pretty. It's almost like rank(), but with a parameter.

Regards

David Rowley