Question

I have different records with columns Start Date and End Date.
If I sort records in ascending order of start date.
and want to find difference between start date of one row and end date of previous row consecutively.

e.g.
Table: Data
------------
Date1           Date2           
13-DEC-2011     15-DEC-2011      
18-DEC-2011     16-DEC-2011     
21-DEC-2011     24-DEC-2011  

one more query if I have a third column
say ID and I want difference grouped by these ID’s
e.g

              ID          Date1             Date2           
              1     13-DEC-2011     15-DEC-2011      
              1     18-DEC-2011     16-DEC-2011     
              2     21-DEC-2011     24-DEC-2011  
              2     25-JAN-2012     25-FEB-2012
              2     29-FEB-2012     25-MAR-2012


              and I need :  


             ID INTERVAL FREE
              1 15 DEC to 18 DEC
              2 24dec to 25 jan;25 feb to 29 feb

Answer 1

Here is SQLFiddle demo

with t1 as
(
select t.*,
        row_number() over (order by date1) rn 
        from t
)
select t1.date1 as d1,t1.Date2 as d2 ,
t2.Date2 as PreviousDate2,
t1.Date1-t2.Date2 as DIff
from t1
left join t1 t2 on t1.rn=t2.rn+1
order by t1.rn

Here is a query to answer your edited question:

SQLFiddle DEMO

If you need to gather rows for each ID in one comma separated line you should do it on the client side not in SQL.

with t1 as
(
select t.*,
        row_number() over (partition by id order by date1) rn 
        from t
)
select t1.id,
t2.Date2 as PreviousDate2,
t1.date1 as d1
from t1
left join t1 t2 on (t1.rn=t2.rn+1) and (t1.id=t2.id)
where t2.Date2 is not null
order by t1.id,t1.rn