Rolling sum per time interval per group - tsql

Question

Table, data and task as follows.
See SQL-Fiddle-Link for demo-data and estimated results.
create table "data"
(
"item" int
, "timestamp" date
, "balance" float
, "rollingSum" float
)
insert into "data" ( "item", "timestamp", "balance", "rollingSum" ) values
( 1, '2014-02-10', -10, -10 )
, ( 1, '2014-02-15', 5, -5 )
, ( 1, '2014-02-20', 2, -3 )
, ( 1, '2014-02-25', 13, 10 )
, ( 2, '2014-02-13', 15, 15 )
, ( 2, '2014-02-16', 15, 30 )
, ( 2, '2014-03-01', 15, 45 )
I need to get all rows in an defined time interval. The above table doesn't hold a record per item for each possible date - only dates on which changes applied are recorded ( it is possible that there are n rows per timestamp per item )
If the given interval does not fit exactly on stored timestamps, the latest timestamp before startdate ( nearest smallest neighbour ) should be used as start-balance/rolling-sum.
estimated results ( time interval: startdate = '2014-02-13', enddate = '2014-02-20' )
"item", "timestamp" , "balance", "rollingSum"
1 , '2014-02-13' , -10 , -10
1 , '2014-02-15' , 5 , -5
1 , '2014-02-20' , 2 , -3
2 , '2014-02-13' , 15 , 15
2 , '2014-02-16' , 15 , 30
I checked questions like this and googled a lot, but didn't found a solution yet.
I don't think it's a good idea to extend "data" table with one row per missing date per item, thus the complete interval ( smallest date <-----> latest date per item may expand over several years ).
Thanks in advance!

Answer 1

select sum(balance)
from table
where timestamp >= (select max(timestamp) from table where timestamp <= 'startdate')
and timestamp <= 'enddate'
Don't know what you mean by rolling-sum.

Answer 2

here is an attempt. Seems it gives the right result, not so beautiful. Would have been easier in sqlserver 2012+:
declare #from date = '2014-02-13'
declare #to date = '2014-02-20'
;with x as
(
select
item, timestamp, balance, row_number() over (partition by item order by timestamp, balance) rn
from (select item, timestamp, balance from data
union all
select distinct item, #from, null from data) z
where timestamp <= #to
)
, y as
(
select item,
timestamp,
coalesce(balance, rollingsum) balance ,
a.rollingsum,
rn
from x d
cross apply
(select sum(balance) rollingsum from x where rn <= d.rn and d.item = item) a
where timestamp between '2014-02-13' and '2014-02-20'
)
select item, timestamp, balance, rollingsum from y
where rollingsum is not null
order by item, rn, timestamp
Result:
item timestamp balance rollingsum
1 2014-02-13 -10,00 -10,00
1 2014-02-15 5,00 -5,00
1 2014-02-20 2,00 -3,00
2 2014-02-13 15,00 15,00
2 2014-02-16 15,00 30,00