SQL> with t1 as (
  2             select 'A' c1, 100 c2 from dual union all
  3             select 'B' c1,    900  c2 from dual union all
  4             select 'C' c1,    2589  c2 from dual union all
  5             select 'D' c1,    123.6598  c2 from dual union all
  6             select 'E' c1,    900  c2 from dual union all
  7             select 'F' c1,    120  c2 from dual union all
  8             select 'G' c1,    100  c2 from dual
  9            )
 10   select C1,C2, dense_rank() over (order by C2 DESC) MC FROM T1
 11  /
 
C1         C2         MC
-- ---------- ----------
C        2589          1
E         900          2
B         900          2
D    123.6598          3
F         120          4
A         100          5
G         100          5
 
7 rows selected

------解决方案--------------------
可见，你的计算并非是2个值都是100，可能有小数位上的误差


------解决方案--------------------
听说这是oracle的一个bug

------解决方案--------------------
没有发现类似的情况


create table t1(c1 varchar2(10),c2 number);
insert into t1 select 'A',100 from dual;
insert into t1 select 'B',900 from dual;
insert into t1 select 'C',2589 from dual;
insert into t1 select 'D',123.6598 from dual;
insert into t1 select 'E',900 from dual;
insert into t1 select 'F',120 from dual;
insert into t1 select 'G',100 from dual;

select c1,c2,dense_rank()over(order by c2 desc) from t1

C	2589	1
E	900	2
B	900	2
D	123.6598	3
F	120	4
A	100	5
G	100	5