Here is one approach you could use:SELECT tMain.YearNum, tMain.SchoolID, tMain.NumberFld,
(tMain.NumberFld)/(tSum1.SumNum) AS NumRatio,
tSum1.SumNum AS CurrentYr_Aggregate,
tSum2.SumNum AS PriorYr_Aggregate,
tSum1.SumNum - tSum2.SumNum AS Aggregate_Diff
FROM tbl_Aggregate_Test AS...