Question

I have two tables sharing a common key ‘itemID’, the first table holds the latest price for each itemID, the second table holds the date and value of any change to the itemID price as shown. The tLatestItemPrice table is automatically updated whenever a new entry is put into tPriceHistory table

tLatestItemPrice TABLE
itemID,   latestPrice
---------------------
item1     400
item2     75
item3     621


tPriceHistory TABLE
itemID,   PriceChangeDate,  NewPrice
------------------------------------
item1     Jan 8th 2012      400
item1     Jan 7th 2012      300
item1     Jan 6th 2012      280
item1     Jan 3rd 2012      270
item2     Jan 8th 2012      75
item2     Jan 5th 2012      72
item2     Jan 1st 2012      60
item3     Jan 7th 2012      621
item3     Jan 6th 2012      601
item3     Jan 2nd 2012      598

I want to have a query that returns me the price difference between the latest price in tLatestItemPrice table and the price of the item at a specific date. ie if I ask for the change of price between the latest and the 4th of January I want a query to return the following dataset

itemID    Price change from 4th Jan
--------------------------
item1     130  (i.e. 400-270)
item2      15
item3      23

Running mysql Ver 14.14 Distrib 5.5.29, for Linux (x86_64)

Answer 1

This is what it become when you have date stored as string in the table

SELECT  a.ITEMID, 
        d.NewPrice - a.NEWPRICE `Price change from 4th Jan`
FROM    tPriceHistory a
        INNER JOIN
        (
          SELECT itemID, 
                 DATE_FORMAT(MAX(STR_TO_DATE(PriceChangeDate, '%b %D %Y')),'%b %D %Y') maxDate
          FROM   tPriceHistory
          WHERE  STR_TO_DATE(PriceChangeDate, '%b %D %Y') <= '2012-01-04'
          GROUP BY itemID
        ) AsOfDatePrice ON a.itemID = AsOfDatePrice.ItemID AND
                           a.PriceChangeDate = AsOfDatePrice.MaxDate
        INNER JOIN 
        (
          SELECT itemID, 
                 DATE_FORMAT(MAX(STR_TO_DATE(PriceChangeDate, '%b %D %Y')),'%b %D %Y') maxDate
          FROM   tPriceHistory
          GROUP BY itemID 
        ) LatestDate ON a.itemID = LatestDate.itemid
        INNER JOIN tPriceHistory d
            ON LatestDate.itemid = d.itemID AND
               LatestDate.maxDate = d.PriceChangeDate

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