Condense Time Periods with SQL

Question

I have a large data set which for the purpose of this question has 3 fields:

• Group Identifier
• From Date
• To Date

On any given row t

Answer 1

; with 
cte as
(
    select  *, rn = row_number() over (partition by [Group ID] order by [From Date])
    from    tbl
),
rcte as
(
    select  rn, [Group ID], [From Date], [To Date], GrpNo = 1, GrpFrom = [From Date], GrpTo = [To Date]
    from    cte
    where   rn  = 1

    union all

    select  c.rn, c.[Group ID], c.[From Date], c.[To Date], 
        GrpNo = case    when    c.[From Date] between r.GrpFrom and dateadd(day, 1, r.GrpTo)
                or  c.[To Date]   between r.GrpFrom and r.GrpTo
                then    r.GrpNo
                else    r.GrpNo + 1
                end,
        GrpFrom= case   when    c.[From Date] between r.GrpFrom and dateadd(day, 1, r.GrpTo)
                or  c.[To Date]   between r.GrpFrom and r.GrpTo
                then    case when c.[From Date] > r.GrpFrom then c.[From Date] else r.GrpFrom end
                else    c.[From Date] 
                end,
        GrpTo  = case   when    c.[From Date] between r.GrpFrom and dateadd(day, 1, r.GrpTo)
                or  c.[To Date]   between r.GrpFrom and dateadd(day, 1, r.GrpTo)
                then    case when c.[To Date] > r.GrpTo then c.[To Date] else r.GrpTo end
                else    c.[To Date]  
                end

    from    rcte r
        inner join cte c    on  r.[Group ID]    = c.[Group ID]
                    and r.rn        = c.rn - 1
)
select  [Group ID], min(GrpFrom), max(GrpTo)
from    rcte
group by [Group ID], GrpNo