On Sun, Sep 21, 2014 at 9:27 PM, Pavel Stehule <pavel.steh...@gmail.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
