There's no need to create the helper field for this purpose, you just can do this:
Select AVG(Sales)from Sales_Home where
Region = ? and
SalesType = ? and
YEAR(\"Date\") = ?
Another way is, compare date to range of dates like
\"Date\" >= ? AND \"Date\" <=?
Date(1;1;Year(Date)) ; Date(12;31;Year(Date))
(There is 'BETWEEN' keyword for this, but very slow on FM.)
I didn't compare this with calculating YEAR() in SQL, but may be faster.