CREATE TABLE #temp
(
id INT IDENTITY(1,1),
month VARCHAR(50),
sales INT,
running_total INT
)

INSERT INTO #temp(month,sales)
SELECT month,sales FROM Table_Monthly_Sales

DECLARE @count INT
DECLARE @max_count INT
DECLARE @total_till_current_month INT

SET @total_till_current_month = 0
SET @count = 1
SELECT @max_count = MAX(id) FROM #temp

WHILE(@count <= @max_count)
BEGIN
	 
	 DECLARE @sales INT
	
         SELECT @sales = sales FROM #temp WHERE id = @count
	 SET  @total_till_current_month = @total_till_current_month + @sales
	
         UPDATE #temp
	 SET running_total = @total_till_current_month
	 WHERE id = @count
	
         SET @count = @count +1
END

SELECT * FROM #temp