How to union two tables, and make one "override" the other for similar records?

Dialect: SQL 92
I have a list of prices for a stocks on each day in a table. I want to make a table that can override the price for a stock on a particular day. If XYZ is $1 on 1/1/2007 according to my main price table, I might want to substitute in $2 for that day instead in my override table. Then, I'd like to union the two, with the $2 record showing as the price for that day.

Dialect: SQL 92
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

Dialect: SQL 92

SQL:

select

 distinct t1.mktcode, t1.date, case when t2.mktcode is null then t1.price else t2.price end

from

 tblstockprices t1

left join tbloverrideprices t2 on t1.mktcode=t2.mktcode and t1.date=t2.date