How to get the date of each price change in the Northwind database?

[Montana Burr]

The Northwind database product cost history looks like this:

ProductID StartDate EndDate StandardCost
707 2011-05-31 2012-05-29 12.0278
707 2012-05-30 2013-05-29 13.8782
707 2013-05-30 NULL 13.0863

I want to get the start dates of each price change for each product. For example, the query should return "2012-05-30" and "2013-05-30" for ProductID=707.

I have this query:

SELECT t1.ProductID,t2.StartDate,ABS(t2.StandardCost - t1.StandardCost) AS costDifference
FROM productcosthistory t1
JOIN productcosthistory t2
ON t1.ProductID = t2.ProductID
WHERE ABS(t2.StandardCost - t1.StandardCost) > 0
ORDER BY ProductID,t2.StartDate

but for each product it returns duplicate start dates for each product. For example, it lists "2011-05-31" twice for product ID 707.