Elimine e reduza intervalos de datas sobrepostos

-- sample table with data
declare @t table(UserID int, StartDate datetime, EndDate datetime)
insert @t select
1, '20110101', '20110102' union all select
1, '20110101', '20110110' union all select
1, '20110108', '20110215' union all select
1, '20110220', '20110310' union all select
2, '20110101', '20110120' union all select
2, '20110115', '20110125'

-- your query starts below

select UserID, Min(NewStartDate) StartDate, MAX(enddate) EndDate
from
(
    select *,
        NewStartDate = t.startdate+v.number,
        NewStartDateGroup =
            dateadd(d,
                    1- DENSE_RANK() over (partition by UserID order by t.startdate+v.number),
                    t.startdate+v.number)
    from @t t
    inner join master..spt_values v
      on v.type='P' and v.number <= DATEDIFF(d, startdate, EndDate)
) X
group by UserID, NewStartDateGroup
order by UserID, StartDate

1. Substituir @t com o nome da sua tabela