SQL: Merge Date Ranges

Question

I\'ve a table, which describes work slices of a business working calendar: (date format is 24 hours format)

PK  | STARTDATE          | ENDDATE
______________         


        

Answer 1

Here's an example using SQL Server syntax. First it determines the "heads", or rows that have no previous overlapping rows. To determine the last "child" of a "head", it looks for the last row that is smaller than the next "head". Here's the SQL:

; with  heads as
        (
        select  row_number() over (order by head.StartDate) as PK
        ,       *
        from    YourTable head
        where   not exists 
                (
                select  *
                from    YourTable prev
                where   prev.StartDate < head.StartDate
                        and head.StartDate < prev.EndDate
                )
        )
select  row_number() over (order by h.StartDate) as PK
,       h.StartDate
,       max(yt.EndDate) as EndDate
from    heads h
left join
        heads nh
on      nh.PK = h.PK + 1
left join
        YourTable yt
on      h.StartDate <= yt.StartDate
        and (yt.StartDate < nh.StartDate or nh.StartDate is null)
group by
        h.StartDate

Live example at SQL Fiddle.