....time.........................quote....dif
| 2008-03-17 00:00:00 | 15000 | |
| 2008-03-17 00:30:00 | 15002 | |
| 2008-03-17 01:00:00 | 15004 | |
| 2008-03-17 01:30:00 | 15006 | |
| 2008-03-17 02:00:00 | 15008 | |
| 2008-03-17 02:30:00 | 15010 | |
| 2008-03-17 03:00:00 | 15008 | |
| 2008-03-17 03:30:00 | 15006 | |
| 2008-03-17 04:00:00 | 15004 | |
| 2008-03-17 04:30:00 | 15002 | |
| 2008-03-17 05:00:00 | 15000 | |
| 2008-03-17 05:30:00 | 15002 | |
| 2008-03-17 06:00:00 | 15004 | |
| 2008-03-17 06:30:00 | 15006 | |
| 2008-03-17 07:00:00 | 15008 | |
..etc etc

i wanna work out the difference in the quote from the previous 30 minutes and store it in the diff field


e.g

| 2008-03-17 00:00:00 | 15000 | null |
| 2008-03-17 00:30:00 | 15002 | 2 |
| 2008-03-17 01:00:00 | 15004 | 2 |
..etc.etc

can i do this with a stored procedure or something?