SQL Server 2014 Merging Overlapping Date Ranges - sql-server

Question

I have a table with 200.000 rows in a SQL Server 2014 database looking like this:
CREATE TABLE DateRanges
(
Contract VARCHAR(8),
Sector VARCHAR(8),
StartDate DATE,
EndDate DATE
);
INSERT INTO DateRanges (Contract, Sector, StartDate, Enddate)
SELECT '111', '999', '01-01-2014', '03-31-2014'
union
SELECT '111', '999', '04-01-2014', '06-30-2014'
union
SELECT '111', '999', '07-01-2014', '09-30-2014'
union
SELECT '111', '999', '10-01-2014', '12-31-2014'
union
SELECT '111', '888', '08-01-2014', '08-31-2014'
union
SELECT '111', '777', '08-15-2014', '08-31-2014'
union
SELECT '222', '999', '01-01-2014', '03-31-2014'
union
SELECT '222', '999', '04-01-2014', '06-30-2014'
union
SELECT '222', '999', '07-01-2014', '09-30-2014'
union
SELECT '222', '999', '10-01-2014', '12-31-2014'
union
SELECT '222', '666', '11-01-2014', '11-30-2014'
UNION
SELECT '222', '555', '11-15-2014', '11-30-2014';
As you can see there can be multiple overlaps for each contract and what I would like to have is the result like this
Contract Sector StartDate EndDate
---------------------------------------------
111 999 01-01-2014 07-31-2014
111 888 08-01-2014 08-14-2014
111 777 08-15-2014 08-31-2014
111 999 09-01-2014 12-31-2014
222 999 01-01-2014 10-31-2014
222 666 11-01-2014 11-14-2014
222 555 11-15-2014 11-30-2014
222 999 12-01-2014 12-31-2014
I can not figure out how this can be done and the examples i have seen on this site quite do not fit my problem.

Answer 1

This answer makes use of a few different techniques. The first is a recursive-cte that creates a table with every relevant cal_date which then gets cross apply'd with unique Contract values to get every combination of both values. The second is window-functions such as lag and row_number to determine a variety of things detailed in the comments below. Lastly, and probably most importantly, gaps-and-islands to determine when one Contract/Sector combination ends and the next begins.
Answer:
--determine range of dates
declare #bgn_dt date = (select min(StartDate) from DateRanges)
, #end_dt date = (select max(EndDate) from DateRanges)
--use a recursive CTE to create a record for each day / Contract
; with dates as
(
select #bgn_dt as cal_date
union all
select dateadd(d, 1, a.cal_date) as cal_date
from dates as a
where a.cal_date < #end_dt
)
select d.cal_date
, c.Contract
into #contract_dates
from dates as d
cross apply (select distinct Contract from DateRanges) as c
option (maxrecursion 0)
--Final Select
select f.Contract
, f.Sector
, min(f.cal_date) as StartDate
, max(f.cal_date) as EndDate
from (
--Use the sum-over to obtain the Island Numbers
select dr.Contract
, dr.Sector
, dr.cal_date
, sum(dr.IslandBegin) over (partition by dr.Contract order by dr.cal_date asc) as IslandNbr
from (
--Determine if the record is the start of a new Island
select a.Contract
, a.Sector
, a.cal_date
, case when lag(a.Sector, 1, NULL) over (partition by a.Contract order by a.cal_date asc) = a.Sector then 0 else 1 end as IslandBegin
from (
--Determine which Contract/Date combinations are valid, and rank the Sectors that are in effect
select cd.cal_date
, dr.Contract
, dr.Sector
, dr.EndDate
, row_number() over (partition by dr.Contract, cd.cal_date order by dr.StartDate desc) as ConractSectorRnk
from #contract_dates as cd
left join DateRanges as dr on cd.Contract = dr.Contract
and cd.cal_date between dr.StartDate and dr.EndDate
) as a
where a.ConractSectorRnk = 1
and a.Contract is not null
) as dr
) as f
group by f.Contract
, f.Sector
, f.IslandNbr
order by f.Contract asc
, min(f.cal_date) asc
Output:
+----------+--------+------------+------------+
| Contract | Sector | StartDate | EndDate |
+----------+--------+------------+------------+
| 111 | 999 | 2014-01-01 | 2014-07-31 |
| 111 | 888 | 2014-08-01 | 2014-08-14 |
| 111 | 777 | 2014-08-15 | 2014-08-31 |
| 111 | 999 | 2014-09-01 | 2014-12-31 |
| 222 | 999 | 2014-01-01 | 2014-10-31 |
| 222 | 666 | 2014-11-01 | 2014-11-14 |
| 222 | 555 | 2014-11-15 | 2014-11-30 |
| 222 | 999 | 2014-12-01 | 2014-12-31 |
+----------+--------+------------+------------+