Perform calculation in SQL using Previous Row values in SQL Server

Try this

 SQL

QuantityDECLARE @Table  TABLE(SalesDate DATETIME, Quantity INT)
INSERT INTO @Table  VALUES('01/jan/2012',50)
INSERT INTO @Table  VALUES ('01/jan/2013', 60)
INSERT INTO @Table  VALUES('01/jan/2014' ,70)

SELECT ROW_NUMBER() OVER
      (
            ORDER BY SalesDate ASC
      )AS RowNumber,
SalesDate, Quantity 
INTO #tmp
FROM @Table

SELECT 
DATEPART(YEAR,SalesDate) AS [Year],
Quantity AS [Sales],
CAST((Quantity - (SELECT Quantity FROM #tmp AS [tbl2] WHERE [tbl2].RowNumber = tbl.RowNumber - 1)) AS DECIMAL(10, 2))/ Quantity
FROM #tmp AS tbl

DROP TABLE #tmp

Output

Year    Sales    (No column name) 2012    50       NULL 2013    60       0.1666666666666 2014    70       0.1428571428571