SQL Server Query to group sequential dates

Question

I have a table named Absence Details and I want to group sequential dates. Here, is the data

EID        AbsenceType  AbsenceStartDate               AbsenceEndDate
769     Holiday     2012-06-25  00:00:00.000            2012-06-25 23:59:59.000
769     Holiday     2012-06-26  00:00:00.000            2012-06-26 23:59:59.000
769     Holiday     2012-09-03  00:00:00.000            2012-09-03 23:59:59.000
769     Holiday     2012-09-04  00:00:00.000            2012-09-04 23:59:59.000
769     Holiday     2012-09-05  00:00:00.000            2012-09-05 23:59:59.000
769     Holiday     2012-09-06  00:00:00.000            2012-09-06 23:59:59.000
769     Holiday     2012-09-07  00:00:00.000            2012-09-07 23:59:59.000

The result i am trying to get is

EID     AbsenceType AbsenceStartDate          AbsenceEndDate
769     Holiday     2012-06-25  00:00:00.000         2012-06-26 23:59:59.000
769     Holiday     2012-09-03  00:00:00.000         2012-09-07 23:59:59.000

Any help is much appreciated.

Answer 1

I have simplified your scenario to isolate the main problem. Let's supose this table with gaps:

with ns as (
select 1 as n union
select 2 as n union
select 3 as n union
select 8 as n union    --gap
select 9 as n )
select * 
into #ns
from ns;

Now, the result you are expecting for is:

ini fi 
--- -- 
1   3  
8   9  

To get this results I massage the data in this way: first I create two views with start and end periods and second, I join both views to get final result. Notice that I join table with it self to locate starts and ends periods:

with 
inis as                                     -- identifying start periods
(
   select n1.n
   from #ns n1
   left outer join #ns n2 
       on n1.n = n2.n + 1
   where n2.n is null
   ),
fis as                                      -- identifying ends periods
( 
   select n1.n 
   from #ns n1
   left outer join #ns n2 
       on n1.n = n2.n - 1
   where n2.n is null
   )  
select inis.n as ini, min( fis.n ) as fi    -- joining starts and ends
from inis 
inner join fis 
  on inis.n <= fis.n
group by inis.n
;
   

You can transfer this technique to your data and data types. If you have any issue translating query be free to ask.

Check query and results.