ถามผู้รู้หน่อยนะครับ โค้ด SQL อยากเอาสองผลรวมมาหารกันครับ

SELECT 
  (@a:=SUM(sc_gpa*sub_credit)) AS All_result, #ผลรวมเกรด
  (@b:=SUM(sub_credit)) AS CreditTotal # ผลรวมหน่วยกิต
  (@a / @b) AS Div_Result # ผลหาร
FROM school

select All_result, CreditTotal, (All_result / CreditTotal) as Div_Result from (
  SELECT 
    SUM(sc_gpa*sub_credit) AS All_result,
    SUM(sub_credit) AS CreditTotal,   @a as testA, @b as testB
  FROM(
    SELECT 20 as sc_gpa, 25 as sub_credit union ALL
    SELECT 25, 30 union ALL
    SELECT 25, 30 union ALL
    SELECT 25, 30
  )as school
) as temp