Here's an interim solution for you. It's horribly inefficient however, I'm working on a second more efficient version.

DROP TABLE #ROCdata

CREATE TABLE #ROCdata (ID INT NOT NULL, Symbol_Code VARCHAR(12), Transaction_date DATE, Close_Price MONEY)

INSERT INTO #ROCdata (ID, Symbol_Code, Transaction_date, Close_Price)

SELECT 1, 'Company1', '28-Apr-10', 11045.27 UNION ALL --

SELECT 2, 'Company1', '29-Apr-10', 11167.32 UNION ALL --

SELECT 3, 'Company1', '30-Apr-10', 11008.61 UNION ALL --

SELECT 4, 'Company1', '3-May-10', 11151.83 UNION ALL --

SELECT 5, 'Company1', '4-May-10', 10926.77 UNION ALL --

SELECT 6, 'Company1', '5-May-10', 10868.12 UNION ALL --

SELECT 7, 'Company1', '6-May-10', 10520.32 UNION ALL --

SELECT 8, 'Company1', '7-May-10', 10380.43 UNION ALL --

SELECT 9, 'Company1', '10-May-10', 10785.14 UNION ALL --

SELECT 10, 'Company1', '11-May-10', 10748.26 UNION ALL --

SELECT 11, 'Company1', '12-May-10', 10896.91 UNION ALL --

SELECT 12, 'Company1', '13-May-10', 10782.95 UNION ALL --

SELECT 13, 'Company1', '14-May-10', 10620.16 UNION ALL -- -3.85

SELECT 14, 'Company1', '17-May-10', 10625.83 UNION ALL -- -4.85

SELECT 15, 'Company1', '18-May-10', 10510.95 UNION ALL -- -4.52

SELECT 16, 'Company1', '19-May-10', 10444.37 UNION ALL -- -6.34

SELECT 17, 'Company1', '20-May-10', 10068.01 UNION ALL -- -7.86

SELECT 18, 'Company1', '21-May-10', 10193.39 UNION ALL -- -6.21

SELECT 19, 'Company1', '24-May-10', 10066.57 UNION ALL -- -4.31

SELECT 20, 'Company1', '25-May-10', 10043.75 UNION ALL -- -3.24

SELECT 21, 'Company2', '28-Apr-10', 110535.27 UNION ALL --

SELECT 22, 'Company2', '29-Apr-10', 114167.32 UNION ALL --

SELECT 23, 'Company2', '30-Apr-10', 151008.61 UNION ALL --

SELECT 24, 'Company2', '3-May-10', 111551.83 UNION ALL --

SELECT 25, 'Company2', '4-May-10', 10926.77 UNION ALL --

SELECT 26, 'Company2', '5-May-10', 108568.12 UNION ALL --

SELECT 27, 'Company2', '6-May-10', 10520.32 UNION ALL --

SELECT 28, 'Company2', '7-May-10', 10380.43 UNION ALL --

SELECT 29, 'Company2', '10-May-10', 10785.14 UNION ALL --

SELECT 30, 'Company2', '11-May-10', 10748.26 UNION ALL --

SELECT 31, 'Company2', '12-May-10', 10896.91 UNION ALL --

SELECT 32, 'Company2', '13-May-10', 10782.95 UNION ALL --

SELECT 33, 'Company2', '14-May-10', 106420.16 UNION ALL -- -3.72

SELECT 34, 'Company2', '17-May-10', 10625.83 UNION ALL -- -90.69

SELECT 35, 'Company2', '18-May-10', 10510.95 UNION ALL -- -93.04

SELECT 36, 'Company2', '19-May-10', 10444.37 UNION ALL -- -90.64

SELECT 37, 'Company2', '20-May-10', 10068.01 UNION ALL -- -7.86

SELECT 38, 'Company2', '21-May-10', 10193.39 UNION ALL -- -90.61

SELECT 39, 'Company2', '24-May-10', 10066.57 UNION ALL -- -4.31

SELECT 40, 'Company2', '25-May-10', 10043.75 -- -3.24

-- calculate ROC

SELECT

d1.ID,

d1.Symbol_Code,

d1.Transaction_date,

d1.Close_Price,

DateToCompare = DATEADD(dd,-13,d1.Transaction_date),

d13.DateChosen,

ROC = (d1.Close_Price-d13.Close_Price)/(1.000*d13.Close_Price) * 100

FROM #ROCdata d1

OUTER APPLY (

SELECT TOP 1

DateChosen = Transaction_date,

Close_Price

FROM #ROCdata

WHERE Symbol_code = d1.Symbol_Code

AND Transaction_date <= DATEADD(dd,-13,d1.Transaction_date)

ORDER BY Transaction_date DESC) d13

ORDER BY d1.Symbol_Code, d1.Transaction_date