Concatenation of adjacent dates in SQL - sql

Question

I would like to know how to make intersections or concatenations of adjacent date ranges in sql.
I have a list of customer start and end dates, for example (in dd/mm/yyyy format, where 31/12/9999 means the customer is still a current customer).
CustID | StartDate | Enddate |
1 | 01/08/2011|19/06/2012|
1 | 20/06/2012|07/03/2012|
1 | 03/05/2012|31/12/9999|
2 | 09/03/2009|16/08/2009|
2 | 16/01/2010|10/10/2010|
2 | 11/10/2010|31/12/9999|
3 | 01/08/2010|19/08/2010|
3 | 20/08/2010|26/12/2011|
Although the dates in different rows don't overlap, I would consider some of the ranges as a contigous period of time, e.g when the start date comes one day after an end date (for a given customer). Hence I would like to return a query that returns just the intersection of the dates,
CustID | StartDate | Enddate |
1 | 01/08/2011|07/03/2012|
1 | 03/05/2012|31/12/9999|
2 | 09/03/2009|16/08/2009|
2 | 16/01/2010|31/12/9999|
3 | 01/08/2010|26/12/2011|
I've looked at CTE tables, but I can't figure out how to return just one row for one contigous block of dates.

Answer 1

This should work in 2005 forward:
;WITH cte2 AS (SELECT 0 AS Number
UNION ALL
SELECT Number + 1
FROM cte2
WHERE Number < 10000)
SELECT CustID, Min(GroupStart) StartDate, MAX(EndDate) EndDate
FROM (SELECT *
, DATEADD(DAY,b.number,a.StartDate) GroupStart
, DATEADD(DAY,1- DENSE_RANK() OVER (PARTITION BY CustID ORDER BY DATEADD(DAY,b.number,a.StartDate)),DATEADD(DAY,b.number,a.StartDate)) GroupDate
FROM Table1 a
JOIN cte2 b
ON b.number <= DATEDIFF(d, startdate, EndDate)
) X
GROUP BY CustID, GroupDate
ORDER BY CustID, StartDate
OPTION (MAXRECURSION 0)
Demo: SQL Fiddle
You can build a quick table of numbers 0-something large enough to cover the spread of dates in your ranges to replace the cte so it doesn't run each time, indexed properly it will run quickly.

Answer 2

you can do this with recursive common table expression:
with cte as (
select t.CustID, t.StartDate, t.EndDate, t2.StartDate as NextStartDate
from Table1 as t
left outer join Table1 as t2 on t2.CustID = t.CustID and t2.StartDate = case when t.EndDate < '99991231' then dateadd(dd, 1, t.EndDate) end
), cte2 as (
select c.CustID, c.StartDate, c.EndDate, c.NextStartDate
from cte as c
where c.NextStartDate is null
union all
select c.CustID, c.StartDate, c2.EndDate, c2.NextStartDate
from cte2 as c2
inner join cte as c on c.CustID = c2.CustID and c.NextStartDate = c2.StartDate
)
select CustID, min(StartDate) as StartDate, EndDate
from cte2
group by CustID, EndDate
order by CustID, StartDate
option (maxrecursion 0);
sql fiddle demo
Quick performance tests:
Results on 750 rows, small periods of 2 days length:
sql fiddle demo
My query: 300 ms
Goat CO query with CTE: 10804 ms
Goat CO query with table of fixed numbers: 7 ms
Results on 5 rows, large periods:
sql fiddle demo
My query: 1 ms
Goat CO query with CTE: 700 ms
Goat CO query with table of fixed numbers: 36 ms