Found a solution:

[sql]
SELECT MktCode,Date,Price
FROM tblStockPrices as t1
LEFT JOIN tblOverridePrices as t2
ON t1.date=t2.date and t1.mktcode=t2.mktcode
WHERE
T2.ID is null
UNION
SELECT MktCode,Date,Price
FROM tblOverridePrices
[/sql]