Re: proposal: window function - change_number

>>>>> "Pavel" == Pavel Stehule <pavel.stehule@gmail.com> writes:
Pavel> HiPavel> I tried to solve following task:
Pavel> I have a table
Pavel> start, reason, kmPavel> =============Pavel>  2014-01-01 08:00:00, private, 10Pavel>  2014-01-01 09:00:00,
commerc,20Pavel>  2014-01-01 10:00:00, commerc, 20Pavel>  2014-01-01 11:00:00, private, 8
 
Pavel> and I would reduce these rows to
Pavel>  2014-01-01 08:00:00, private, 10Pavel>  2014-01-01 09:00:00, commerc, 20 + 20 = 40Pavel>  2014-01-01 11:00:00,
private,8
 
Pavel> It is relative hard to it now with SQL only.

Only relatively. My standard solution is something like this:

select start_time, reason, sum(km) as km from (select max(label_time) over (order by start) as start_time,
reason,km         from (select start, reason, km,                      case when reason
isdistinct from                                lag(reason) over (order by start)                           then start
                  end as label_time                 from yourtable              ) s2      ) s1group by start_time,
reasonorderby start_time;
 

(Your change_number idea is essentially equivalent to doing
sum(case when x is distinct from lag(x) over w then 1 end) over w,
except that since window functions can't be nested, that expression
requires a subquery.)

-- 
Andrew (irc:RhodiumToad)