T-SQL Subquery Max(Date) and Joins

Try this:

Select *,
    Price = (Select top 1 Price 
             From MyPrices 
             where PartID = mp.PartID 
             order by PriceDate desc
            )
from MyParts mp

Here's another way to do it without subqueries. This method will often outperform others, so it's worth testing both methods to see which gives the best performance.

SELECT
     PRT.PartID,
     PRT.PartNumber,
     PRT.Description,
     PRC1.Price,
     PRC1.PriceDate
FROM
     MyParts PRT
LEFT OUTER JOIN MyPrices PRC1 ON
     PRC1.PartID = PRT.PartID
LEFT OUTER JOIN MyPrices PRC2 ON
     PRC2.PartID = PRC1.PartID AND
     PRC2.PriceDate > PRC1.PriceDate
WHERE
     PRC2.PartID IS NULL

This will give multiple results if you have two prices with the same EXACT PriceDate (Most other solutions will do the same). Also, I there is nothing to account for the last price date being in the future. You may want to consider a check for that regardless of which method you end up using.

SELECT
    MyParts.*,MyPriceDate.Price,MyPriceDate.PriceDate
    FROM MyParts
        INNER JOIN (SELECT Partid, MAX(PriceDate) AS MaxPriceDate FROM MyPrice GROUP BY Partid) dt ON MyParts.Partid = dt.Partid
        INNER JOIN MyPrice ON dt.Partid = MyPrice.Partid AND MyPrice.PriceDate=dt.MaxPriceDate