If I'm understanding correctly, here's what I'd do in this situation:

IF object_id('tempdb..#foo') IS NOT NULL

DROP TABLE #foo;

CREATE TABLE #foo (item CHAR(1), changedate DATETIME, value INT);

INSERT #foo (item, changedate, value)

VALUES ('x', '2014-06-15', 0);

INSERT #foo (item, changedate, value)

VALUES ('y', '2014-06-16', 1);

INSERT #foo (item, changedate, value)

VALUES ('x', '2014-07-10', 1);

INSERT #foo (item, changedate, value)

VALUES ('y', '2014-07-15', 2);

INSERT #foo (item, changedate, value)

VALUES ('x', '2014-08-10', 3);

WITH CTE AS

(

SELECT RN = ROW_NUMBER() OVER(ORDER BY item, changedate),

*

FROM #foo

)

SELECT

-- CTE1.RN,

CTE1.item,

CTE1.value,

CTE1.changedate AS startdate,

ISNULL(DATEADD(day, -1, CTE2.changedate), '9999-12-31') AS enddate

FROM CTE AS CTE1

LEFT OUTER JOIN CTE AS CTE2 ON

CTE1.RN + 1 = CTE2.RN

AND CTE1.item = CTE2.item

;

This way you have a start and end date range for each value of each product that you can query.