i have changed the my input and output and add date and comments to make it clear, for multiple entry with in a month it shall take the last entry.

declare @t table

([id] int,[Amount] int,Sector char(1),Currentdate datetime);

INSERT INTO @t

([id],[Amount],Sector,currentdate )

VALUES

(1, 100,'A','2002-01-01 00:00:00.000'),

(2,50,'A','2002-01-02 00:00:00.000'),

(3, 200,'B','2002-01-01 00:00:00.000'),

(4, 300,'C','2002-01-01 00:00:00.000'),

(5, 400,'A','2002-02-02 00:00:00.000'),

(6, 500,'B','2002-02-02 00:00:00.000'),

(7, 600,'C','2002-02-02 00:00:00.000'),

(8, 100,'A','2002-03-02 00:00:00.000'),

(9, 500,'B','2002-03-03 00:00:00.000'),

(10, 100,'C','2002-03-03 00:00:00.000'),

(11, 200,'C','2002-03-03 00:00:00.000')

select * from @t

declare @output table ([AfterIncreaseDecreaseAmount] int,Amount int,[Monthno]int,sector char(1))

INSERT INTO @output

([AfterIncreaseDecreaseAmount],Amount,[Monthno],sector )

VALUES

(50,100,1,'A'), -- for 01/01/2002 it is 100 , for 02/02/2002 for multiple entry with in the month it shall take the last amount based on max(id) or last(currentdate).

(+350,400,2,'A'), -- for second month 02/02/2002 it 400 > 50 it shows increase from first month so 400-50=+350 so --> AfterIncreaseDecreaseAmount=350

(-300,100,3,'A'), -- for third month 02/03/2002 it is 100 < 400 (sum of 1st,2st month amount) so it shows decrease 100-400=-300 so-->AfterIncreaseDecreaseAmount=-300

(+200,200,1,'B'),-- for b we have 200 it is 200 > 0 ,if it is only one it shall count that as 0 so it shows increase 200-0=200 --> AfterIncreaseDecrease=200

(+300,500,2,'B'),-- for b we have 500 500 > 200--> 500-200=300 it show increased -->AfterIncreaseDecrease=300

(0,500,3,'B'), -- for b in third month 500-500(sum of 1st,2stmonth amount of b) =0 AfterIncreaseDecrease=0

(+300,300,1,'C'),

(+300,600,2,'C'),

(400,100,3,'C')

select * from @output