使用FDQuery的Aggregates属性获取汇总值

 with FDQuery1.Aggregates.Add do begin
  Name := 'Total';
  Expression := 'SUM((ItemPrice + ItemTaxes) * ItemCount)';
  Active := True;
end;
FDQuery1.AggregatesActive := True;
...
Label1.Caption := VarToStr(FDQuery1.Aggregates[0].Value)