Empty field value should be previous field value based on category

[nithyalakshmip]

Hi,

I want the AMOUNT column null should be previous one, and that too based on CATEGORY_ID. The outcome should be like Result column.

P_ID	P_NAME	CATEGORY_ID	AMOUNT	Result
1	pepsi	cool drinks	50	50
2	7up	cool drinks		50
3	maza	cool drinks	40	40
4	slice	cool drinks		40
5	pizza	food	100	100
6	sprite	cool drinks		40
7	burger	food	50	50
8	sandwich	food	70	70