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.

Answer 2

This is my solution.

IF OBJECT_ID('tempdb..#tblDates') IS NOT NULL
    DROP TABLE #tblDates

CREATE TABLE #tblDates (AutoId INT IDENTITY, StartDate DATE, EndDate DATE)

INSERT #tblDates (StartDate, EndDate) SELECT '2014-11-02', '2014-11-08'
INSERT #tblDates (StartDate, EndDate) SELECT '2014-11-07', '2014-11-10'
INSERT #tblDates (StartDate, EndDate) SELECT '2014-11-06', '2014-11-12'

INSERT #tblDates (StartDate, EndDate) SELECT '2014-11-02', '2014-11-15'

INSERT #tblDates (StartDate, EndDate) SELECT '2014-12-10', '2014-12-13'
INSERT #tblDates (StartDate, EndDate) SELECT '2014-12-12', '2014-12-15'
INSERT #tblDates (StartDate, EndDate) SELECT '2014-12-14', '2014-12-16'


-- Optional / Remove the duplicated records of same StartDate and EndDate
DELETE FROM #tblDates WHERE AutoId NOT IN (SELECT MAX(AutoId) FROM #tblDates GROUP BY StartDate, EndDate)

-- Optional / Get only the record with max EndDate grouped by StartDate, Remove Others
DELETE  d1
FROM    #tblDates d1
        JOIN (SELECT x.StartDate, MAX(x.EndDate) MAXEndDate FROM #tblDates x GROUP BY x.StartDate) d2 ON d2.StartDate = d1.StartDate AND d2.MAXEndDate != d1.EndDate

-- Optional / Get only the record with min StartDate grouped by EndDate, Remove Others
DELETE  d1
FROM    #tblDates d1
        JOIN (SELECT x.EndDate, MIN(x.StartDate) MINStartDate FROM #tblDates x GROUP BY x.EndDate) d2 ON d2.EndDate = d1.EndDate AND d2.MINStartDate != d1.StartDate

-- Optional / Remove the overlapping ranges of relevant StartDate and EndDate
DELETE  c
FROM    #tblDates p
        JOIN #tblDates c ON c.AutoId != p.AutoId AND c.StartDate BETWEEN p.StartDate AND p.EndDate AND c.EndDate BETWEEN p.StartDate AND p.EndDate


;WITH Ranges
AS
(
    SELECT  s.StartDate, s.EndDate
    FROM    #tblDates s
            LEFT JOIN #tblDates a ON a.AutoId != s.AutoId AND s.StartDate BETWEEN a.StartDate AND a.EndDate AND s.StartDate != a.StartDate
    WHERE   a.AutoId IS NULL
    UNION ALL
    SELECT  r.StartDate, d.EndDate
    FROM    Ranges r
            JOIN #tblDates d ON r.EndDate != d.EndDate AND r.EndDate BETWEEN d.StartDate AND d.EndDate
)

SELECT StartDate, MAX(EndDate) EndDate FROM Ranges GROUP BY StartDate

Answer 3

Based on ErikE response :

IF(object_id('dbo.Periods') is not null)
    drop table Periods

go
create table Periods (
    StartDate date not null,
    EndDate date not null
)
go
insert into Periods(StartDate,EndDate)
select '1980-01-01','1980-01-10' union all
select '1980-01-03','1980-01-07' union all

select '2000-01-01','2000-01-10' union all
select '2000-01-05','2000-01-30' union all
select '2000-01-12','2000-01-20' union all

select '2021-01-01','2021-01-01'
go

; with LeadLag AS (
   SELECT     
     rownum = row_number() OVER( ORDER BY StartDate),
     PrevEndDate = Coalesce(Lag(EndDate) OVER (ORDER BY StartDate), Convert(datetime2, '0001-01-01')), 
     p.*
   FROM Periods p
), Dates AS (
   SELECT
        StartDate = CASE WHEN PrevEndDate < StartDate THEN StartDate ELSE NULL END,
        EndDate,           
        rownum
   FROM   LeadLag
), startGrouping AS (
   SELECT
      StartDate =  max(StartDate) OVER (ORDER BY rownum rows UNBOUNDED PRECEDING),
      EndDate,
      rownum
   FROM Dates
),
 groups AS (
   SELECT
      StartDate,
      EndDate,
      rownum,
      ingroupRownum = row_number() OVER(PARTITION BY StartDate ORDER BY EndDate desc)
   FROM startGrouping e1
)
SELECT StartDate, EndDate
from groups
WHERE  ingroupRownum = 1