Question

select pdat
      ,ac_no
      , bal
 from dummy_table;

PDATE     | AC_NO | BAL  
========================    
14/02/2012| abcd  | 1200    
15/02/2012| abcd  | 1300
27/02/2012| abcd  | 1300

My question is how can I make the result set to fetch

PDATE     | AC_NO | BAL  | Difference
=====================================
14/02/2012| abcd  | 1200 |    8
22/02/2012| abcd  | 1300 |    7

Difference of row one is calculated on the basis of 22/02/2012-14/02/2012.
But how can I calculate the difference of last date from the month’s ending date? in this case 29th Feb 2012

Answer 1

Key here is LEAD function – read documentation about analytical functions in SQL, they are very powerful tool. LEAD(pdat) OVER (ORDER BY pdat) gets you value of pdat in next row which comes in ascending order of pdat. Then nvl function takes care about last date where there is no next date – it uses last_day(pdat) function call which returns last day of a month pdat falls in.

If you want to handle several months – you will need to do some analysis with DECODE function to detect if pdat and LEAD(pdat) OVER (ORDER BY pdat) belong to the same month, and to use either LEAD(pdat) OVER (ORDER BY pdat) or last_day(pdat) as date you are comparing against. I will leave it as your homework.

 create table dummy_table(pdat date, ac_no varchar2(10), bal number);

 insert into dummy_table values ('14-FEB-2012', 'abcd', 1200);
 insert into dummy_table values ('15-FEB-2012', 'abcd', 1300);
 insert into dummy_table values ('27-FEB-2012', 'abcd', 1300);

 select pdat
     , ac_no
     , bal
     , nvl(lead(pdat) over (order by pdat) - pdat, last_day(pdat) - pdat) 
  from dummy_table;

 2/14/2012 abcd 1200  1
 2/15/2012 abcd 1300 12
 2/27/2012 abcd 1300  2