Finding occupied dates from date ranges - sql

Question

This is the data that I have on SQL-Sever 2005 mode
ID FromDate ToDate Diff
ZIM145876-01 03/01/2011 02/29/2012 1
ZIM145876-01 03/01/2012 02/28/2013 1
ZIM145876-01 03/01/2013 02/28/2014 NULL
ZIM145881-02 02/01/2012 03/31/2012 1
ZIM145881-02 04/01/2012 06/30/2012 1
ZIM145881-02 07/01/2012 09/30/2012 1
ZIM145881-02 10/01/2012 03/31/2013 1
ZIM145881-02 04/01/2013 06/30/2013 NULL
ZIM145878-01 05/15/2010 05/14/2011 201
ZIM145878-01 12/01/2011 11/30/2012 1
ZIM145878-01 12/01/2012 11/30/2013 NULL
Now on first case I want to have
ZIM145876-01 03/01/2011 02/28/2014
ZIM145881-02 02/01/2012 06/30/2013
However in 3rd case we have two occupied dates for same IDs and this is what I want
ZIM145878-01 05/15/2010 05/14/2011
ZIM145878-01 12/01/2011 11/30/2013
So any hint would be highly appreciated
(SQLFiddle)

Answer 1

This seems to do the job. Unfortunately, it's not going to be too efficient on large data sets:
declare #t table (ID varchar(50),FromDate date,ToDate date, Diff int)
insert into #t(ID,FromDate,ToDate,Diff) values
('ZIM145876-01','20110301','20120229',1 ),
('ZIM145876-01','20120301','20130228',1 ),
('ZIM145876-01','20130301','20140228',NULL),
('ZIM145881-02','20120201','20120331',1 ),
('ZIM145881-02','20120401','20120630',1 ),
('ZIM145881-02','20120701','20120930',1 ),
('ZIM145881-02','20121001','20130331',1 ),
('ZIM145881-02','20130401','20130630',NULL),
('ZIM145878-01','20100515','20110514',201 ),
('ZIM145878-01','20111201','20121130',1 ),
('ZIM145878-01','20121201','20131130',NULL)
;With Islands as (
select ID,FromDate,ToDate from #t t1 where not exists (select * from #t t2 where t2.ToDate = DATEADD(day,-1,t1.FromDate) and t1.ID = t2.ID)
union all
select i.ID,i.FromDate,t.ToDate
from
Islands i
inner join
#t t
on
i.ID = t.ID and
i.ToDate = DATEADD(day,-1,t.FromDate)
)
select ID,FromDate,MAX(ToDate) from Islands
group by ID,FromDate
Results:
ID FromDate
-------------------------------------------------- ---------- ----------
ZIM145878-01 2010-05-15 2011-05-14
ZIM145876-01 2011-03-01 2014-02-28
ZIM145878-01 2011-12-01 2013-11-30
ZIM145881-02 2012-02-01 2013-06-30
It could be more efficient if the occupancy was recorded as a semi-open interval (e.g. Inclusive FromDate, Exclusive ToDate) - because then we wouldn't have to call DATEADD to find the place where the periods join together.
I don't know what the Diff column is about, and I haven't used it.