Get Last Change Date in SQL - sql

Question

StockCode TimeStamp UnitPrice TrnDate
---------------------------------------------------------------
360120 0x000000000DBE9EED 16.8000 2015-02-13 0:00:00.000
360120 0x000000000DBEE175 16.8000 2015-02-17 00:00:00.000
360120 0x000000000DC177AC 16.8000 2015-02-24 00:00:00.000
360120 0x000000000DC1EEFB 16.0000 2015-02-25 00:00:00.000
360120 0x000000000DC1E9BE 16.8000 2015-02-25 00:00:00.000
360120 0x000000000DC261CE 16.0000 2015-02-27 00:00:00.000
360120 0x000000000DC2628D 16.8000 2015-02-27 00:00:00.000 **
360120 0x000000000DFC46EA 16.8000 2015-03-02 00:00:00.000
The query needs to return 2015-02-27 where price went form 16.000 to 16.8000,
which is the last price change date for this item in the price history table.
The way I wrote it, the query it only works if the price didn't change back to a price that exists in the table for that stock code.
SELECT
MAX(MinTrnDate) as LastTrnDate
FROM
(SELECT
a.UnitPrice, MIN(TrnDate) as MinTrnDate
FROM
(SELECT
ph.StockCode,
CONVERT(DECIMAL(18,4), (ph.InvoiceValue/ ph.InvoiceQty)) as UnitPrice,
ph.TrnDate
FROM
ArSalesMove ph
JOIN
ArCustomer c WITH(NOLOCK) on ph.Customer = c.Customer
WHERE
LTRIM(RTRIM(ph.StockCode)) = '360120'
AND (ph.InvoiceValue/ ph.InvoiceQty) > 0
AND c.PriceCode = 'A') AS a
WHERE
a.UnitPrice > 0
GROUP BY a.UnitPrice) AS b
HAVING
MIN(CONVERT(DECIMAL(18, 2), UnitPrice)) <> MAX(CONVERT(DECIMAL(18, 2), UnitPrice)) --Used to exclude stockcodes with no change...

Answer 1

For SQL Server 2012 or newer:
;WITH
cte AS
(
SELECT *,
LAG(UnitPrice, 1) OVER (PARTITION BY StockCode ORDER BY TrnDate) AS LastPrice
FROM PriceHistory
)
SELECT StockCode, MAX(TrnDate)
FROM cte
WHERE UnitPrice != LastPrice
GROUP BY StockCode
For SQL Server 2005 and up:
;WITH
cte AS
(
SELECT *,
ROW_NUMBER() OVER (PARTITION BY StockCode ORDER BY TrnDate) AS RowNumber
FROM PriceHistory
)
SELECT c2.StockCode, MAX(c2.TrnDate)
FROM cte c1
INNER JOIN cte c2 ON c1.StockCode = c2.StockCode AND c1.RowNumber + 1 = c2.RowNumber
WHERE c1.UnitPrice != c2.UnitPrice
GROUP BY c2.StockCode