关于零售业中的二八sql统计

零售业里的，20%商品，产生80%的销售，就是要查出是哪些商品的贡献度最大。
如图：
我如何取每类商品中按销售额降序排列后，依次累加直到累计值>=0.8*sum（销售额）的前几种商品的销售额明细。

sql：
-- Create table
create table TAB_1
(
  COD VARCHAR2(10),
  REC NUMBER(8,4)
);
insert into TAB_1 (COD, REC)
values ('A', 500);
insert into TAB_1 (COD, REC)
values ('B', 600);
insert into TAB_1 (COD, REC)
values ('C', 200);
insert into TAB_1 (COD, REC)
values ('D', 223);
insert into TAB_1 (COD, REC)
values ('D', 30);
insert into TAB_1 (COD, REC)
values ('A', 55);
insert into TAB_1 (COD, REC)
values ('A', 3);
commit;

elect m.R1, m.Cod, m.Rec1, n.Rec1 需要查询的数量, n.Cod 需要查询的代码
  From (Select Cod, Rec1, Rownum R1
          From (Select Cod, Rec1
                  From (Select Cod, Sum(Rec) Rec1 From Tab_1 a Group By Cod)
                 Order By Rec1)) m,
       (Select Cod, Rec1, Rownum R2
          From (Select Cod, Rec1
                  From (Select Cod, Sum(Rec) Rec1 From Tab_1 a Group By Cod)
                 Order By Rec1)) n
Where m.R1 <= n.R2
   And m.R1 =
       (Select Max(R1)
          From (Select m.R1, Sum(n.Rec1)
                  From (Select Cod, Rec1, Rownum R1
                          From (Select Cod, Rec1
                                  From (Select Cod, Sum(Rec) Rec1
                                          From Tab_1 a
                                         Group By Cod)
                                 Order By Rec1)) m,
                       (Select Cod, Rec1, Rownum R2
                          From (Select Cod, Rec1
                                  From (Select Cod, Sum(Rec) Rec1
                                          From Tab_1 a
                                         Group By Cod)
                                 Order By Rec1)) n
                 Where m.R1 <= n.R2
                 Group By m.R1
                Having Sum(n.Rec1) > = (Select Sum(Rec) * 0.8 From Tab_1)))
Order By m.Rec1 Desc