Please help its urgent

i have a data which will be updated daily in database.. my crystal report will show all that data..

Now here is da query on which i need help..

Take an example i have values shown on report.. eg

100

300

500

900

1000

.......

note above values will keep on increasing every day..

OUTPUT should be

300 - 100 = 200

500 - 300= 200 .......

so its like i want difference by subracting second value (D2) minus (D1) and then (D3) minus (D2) and so on till it encounters last row.

ACTUAL VALUES ........ how can i get
FROM DB ....... DIFFERENCE OUTPUT ...... OUTPUT

100 ............... ( 300 minus 100 ) ....... 200

300 ............... ( 500 minus 300 ) ....... 200

500 ............... ( 900 minus 500 ) ....... 400

900 ............... ( 1000 minus 900 ) ....... 100

1000 ............... last row will be blank as below this der is no row

i hope u ppl get ma query and plz help me.. its urgentNext({field}) - {field}