I've inherited a SQL statement that looks approximately like this:

SELECT Sum(tblA.A9A) As [1990],
      Sum(tblA.A9A)/Sum(tblA.I9N) As [1990P],
      Sum(tblA.A0A) As [2000],
      Sum(tblA.A0A)/Sum(tblA.I9N) As [2000P]
FROM [geoB]
INNER JOIN tblA
ON [geoB].STFID = tblA.STFID;

1990,2000 refer to years and the number of years that appears in a
SQL varies. I'm uncomfortable with that Sum(tblA.I9N) which will be
repeated for each year in the SQL. Will the SQL automatically be
optimized so that sum is done only once? If not, how do I compute and
name it for use as an intermediate, but not make it one of the items selected?
           
              thanks, --thelma